feat: add i18n internationalization support

- Add react-i18next, i18next with browser language detection
- Support Traditional Chinese (zh-TW) and English (en)
- Default language: zh-TW, stored in localStorage
- Create 10 translation namespaces (common, auth, dashboard, tasks, etc.)
- Add LanguageSwitcher component in header
- Translate pages: Login, Dashboard, Tasks, Spaces, Workload, Audit

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/public/locales/en/audit.json

@@ -0,0 +1,56 @@
+{
+  "title": "Audit Log",
+  "subtitle": "Track all operations in the system",
+  "filters": {
+    "action": "Action Type",
+    "user": "User",
+    "entity": "Entity Type",
+    "dateRange": "Date Range",
+    "allActions": "All Actions",
+    "allUsers": "All Users",
+    "allEntities": "All Entities"
+  },
+  "actions": {
+    "create": "Create",
+    "update": "Update",
+    "delete": "Delete",
+    "login": "Login",
+    "logout": "Logout",
+    "assign": "Assign",
+    "unassign": "Unassign",
+    "statusChange": "Status Change",
+    "comment": "Comment",
+    "upload": "Upload",
+    "download": "Download"
+  },
+  "entities": {
+    "task": "Task",
+    "project": "Project",
+    "space": "Space",
+    "user": "User",
+    "comment": "Comment",
+    "attachment": "Attachment"
+  },
+  "columns": {
+    "timestamp": "Timestamp",
+    "user": "User",
+    "action": "Action",
+    "entity": "Entity",
+    "details": "Details",
+    "ipAddress": "IP Address"
+  },
+  "details": {
+    "before": "Before",
+    "after": "After",
+    "changes": "Changes"
+  },
+  "export": {
+    "title": "Export Log",
+    "csv": "Export as CSV",
+    "json": "Export as JSON"
+  },
+  "empty": {
+    "title": "No Audit Records",
+    "description": "No audit records match the current filters"
+  }
+}

frontend/public/locales/en/auth.json

@@ -0,0 +1,28 @@
+{
+  "login": {
+    "title": "Login",
+    "subtitle": "Sign in to your account",
+    "email": "Email",
+    "emailPlaceholder": "Enter your email",
+    "password": "Password",
+    "passwordPlaceholder": "Enter your password",
+    "rememberMe": "Remember me",
+    "forgotPassword": "Forgot password?",
+    "submit": "Sign in",
+    "loggingIn": "Signing in...",
+    "noAccount": "Don't have an account?",
+    "signUp": "Sign up"
+  },
+  "errors": {
+    "invalidCredentials": "Invalid email or password",
+    "accountLocked": "Account is locked. Please contact administrator.",
+    "emailRequired": "Email is required",
+    "passwordRequired": "Password is required",
+    "invalidEmail": "Please enter a valid email address",
+    "loginFailed": "Login failed. Please try again later."
+  },
+  "welcome": {
+    "title": "Project Control Center",
+    "subtitle": "Manage your projects, tasks, and teams"
+  }
+}

frontend/public/locales/en/common.json

@@ -0,0 +1,102 @@
+{
+  "buttons": {
+    "save": "Save",
+    "cancel": "Cancel",
+    "delete": "Delete",
+    "edit": "Edit",
+    "create": "Create",
+    "close": "Close",
+    "confirm": "Confirm",
+    "back": "Back",
+    "next": "Next",
+    "previous": "Previous",
+    "submit": "Submit",
+    "reset": "Reset",
+    "search": "Search",
+    "filter": "Filter",
+    "export": "Export",
+    "import": "Import",
+    "refresh": "Refresh",
+    "add": "Add",
+    "remove": "Remove",
+    "view": "View",
+    "download": "Download",
+    "upload": "Upload"
+  },
+  "labels": {
+    "loading": "Loading...",
+    "noData": "No data",
+    "required": "Required",
+    "optional": "Optional",
+    "all": "All",
+    "none": "None",
+    "yes": "Yes",
+    "no": "No",
+    "active": "Active",
+    "inactive": "Inactive",
+    "enabled": "Enabled",
+    "disabled": "Disabled",
+    "actions": "Actions",
+    "details": "Details",
+    "description": "Description",
+    "name": "Name",
+    "title": "Title",
+    "status": "Status",
+    "type": "Type",
+    "date": "Date",
+    "time": "Time",
+    "createdAt": "Created at",
+    "updatedAt": "Updated at",
+    "createdBy": "Created by",
+    "assignee": "Assignee",
+    "selectAssignee": "Select assignee...",
+    "searchUsers": "Search users...",
+    "noUsersFound": "No users found",
+    "typeToSearch": "Type to search users"
+  },
+  "messages": {
+    "success": "Operation successful",
+    "error": "Operation failed",
+    "confirmDelete": "Are you sure you want to delete? This action cannot be undone.",
+    "unsavedChanges": "You have unsaved changes. Are you sure you want to leave?",
+    "networkError": "Network error. Please try again later.",
+    "sessionExpired": "Session expired. Please log in again.",
+    "permissionDenied": "You do not have permission to perform this action.",
+    "notFound": "The requested resource was not found."
+  },
+  "validation": {
+    "required": "This field is required",
+    "email": "Please enter a valid email address",
+    "minLength": "Minimum {{min}} characters required",
+    "maxLength": "Maximum {{max}} characters allowed",
+    "invalidFormat": "Invalid format"
+  },
+  "nav": {
+    "dashboard": "Dashboard",
+    "spaces": "Spaces",
+    "projects": "Projects",
+    "tasks": "Tasks",
+    "workload": "Workload",
+    "health": "Project Health",
+    "audit": "Audit Log",
+    "settings": "Settings",
+    "logout": "Logout"
+  },
+  "language": {
+    "switch": "Switch language",
+    "zhTW": "繁體中文",
+    "en": "English"
+  },
+  "notifications": {
+    "title": "Notifications",
+    "markAllRead": "Mark all as read",
+    "noNotifications": "No notifications",
+    "viewAll": "View all"
+  },
+  "pagination": {
+    "page": "Page {{page}}",
+    "of": "of {{total}}",
+    "showing": "Showing {{from}}-{{to}} of {{total}}",
+    "itemsPerPage": "Items per page"
+  }
+}

frontend/public/locales/en/dashboard.json

@@ -0,0 +1,47 @@
+{
+  "title": "Dashboard",
+  "welcome": "Welcome back, {{name}}",
+  "stats": {
+    "myTasks": "My Tasks",
+    "overdueTasks": "Overdue Tasks",
+    "completedTasks": "Completed Tasks",
+    "activeProjects": "Active Projects",
+    "teamMembers": "Team Members",
+    "totalTasks": "Total Tasks",
+    "pendingReview": "Pending Review"
+  },
+  "sections": {
+    "recentActivity": "Recent Activity",
+    "upcomingDeadlines": "Upcoming Deadlines",
+    "myAssignedTasks": "Tasks Assigned to Me",
+    "projectOverview": "Project Overview",
+    "quickActions": "Quick Actions"
+  },
+  "activity": {
+    "taskCreated": "Created task \"{{task}}\"",
+    "taskCompleted": "Completed task \"{{task}}\"",
+    "taskAssigned": "Assigned task \"{{task}}\" to {{assignee}}",
+    "commentAdded": "Added a comment on task \"{{task}}\"",
+    "projectCreated": "Created project \"{{project}}\"",
+    "noRecentActivity": "No recent activity"
+  },
+  "deadlines": {
+    "today": "Due today",
+    "tomorrow": "Due tomorrow",
+    "thisWeek": "Due this week",
+    "overdue": "Overdue",
+    "noUpcoming": "No upcoming deadlines"
+  },
+  "quickActions": {
+    "createTask": "Create Task",
+    "createProject": "Create Project",
+    "viewAllTasks": "View All Tasks",
+    "viewWorkload": "View Workload"
+  },
+  "widgets": {
+    "tasksByStatus": "Tasks by Status",
+    "tasksByPriority": "Tasks by Priority",
+    "projectProgress": "Project Progress",
+    "teamWorkload": "Team Workload"
+  }
+}

frontend/public/locales/en/health.json

@@ -0,0 +1,48 @@
+{
+  "title": "Project Health",
+  "subtitle": "Monitor the overall health of projects",
+  "overall": {
+    "title": "Overall Health",
+    "healthy": "Healthy",
+    "atRisk": "At Risk",
+    "critical": "Critical"
+  },
+  "metrics": {
+    "schedule": "Schedule",
+    "budget": "Budget",
+    "scope": "Scope",
+    "quality": "Quality",
+    "resources": "Resources"
+  },
+  "status": {
+    "onTrack": "On Track",
+    "delayed": "Delayed",
+    "ahead": "Ahead",
+    "overBudget": "Over Budget",
+    "underBudget": "Under Budget"
+  },
+  "indicators": {
+    "title": "Health Indicators",
+    "taskCompletion": "Task Completion Rate",
+    "onTimeDelivery": "On-time Delivery Rate",
+    "blockedTasks": "Blocked Tasks",
+    "overdueRate": "Overdue Rate",
+    "velocityTrend": "Velocity Trend"
+  },
+  "risks": {
+    "title": "Risks",
+    "high": "High Risk",
+    "medium": "Medium Risk",
+    "low": "Low Risk",
+    "mitigated": "Mitigated"
+  },
+  "actions": {
+    "viewDetails": "View Details",
+    "exportReport": "Export Report",
+    "setAlert": "Set Alert"
+  },
+  "empty": {
+    "title": "No Health Data",
+    "description": "The project needs more data to display health metrics"
+  }
+}

frontend/public/locales/en/projects.json

@@ -0,0 +1,50 @@
+{
+  "title": "Projects",
+  "createProject": "Create Project",
+  "editProject": "Edit Project",
+  "deleteProject": "Delete Project",
+  "projectSettings": "Project Settings",
+  "fields": {
+    "name": "Name",
+    "namePlaceholder": "Enter project name",
+    "description": "Description",
+    "descriptionPlaceholder": "Enter project description",
+    "status": "Status",
+    "startDate": "Start Date",
+    "endDate": "End Date",
+    "owner": "Project Owner",
+    "space": "Space"
+  },
+  "status": {
+    "planning": "Planning",
+    "active": "Active",
+    "on_hold": "On Hold",
+    "completed": "Completed",
+    "cancelled": "Cancelled"
+  },
+  "tabs": {
+    "overview": "Overview",
+    "tasks": "Tasks",
+    "members": "Members",
+    "settings": "Settings",
+    "files": "Files",
+    "activity": "Activity"
+  },
+  "stats": {
+    "totalTasks": "Total Tasks",
+    "completedTasks": "Completed",
+    "inProgress": "In Progress",
+    "overdue": "Overdue",
+    "progress": "Overall Progress"
+  },
+  "messages": {
+    "created": "Project created",
+    "updated": "Project updated",
+    "deleted": "Project deleted",
+    "confirmDelete": "Are you sure you want to delete this project? This will delete all related tasks."
+  },
+  "empty": {
+    "title": "No Projects",
+    "description": "Create your first project to start managing tasks"
+  }
+}

frontend/public/locales/en/settings.json

@@ -0,0 +1,73 @@
+{
+  "title": "Settings",
+  "projectSettings": "Project Settings",
+  "tabs": {
+    "general": "General",
+    "members": "Members",
+    "customFields": "Custom Fields",
+    "notifications": "Notifications",
+    "integrations": "Integrations",
+    "danger": "Danger Zone"
+  },
+  "general": {
+    "title": "General Settings",
+    "projectName": "Project Name",
+    "description": "Description",
+    "status": "Status",
+    "visibility": "Visibility",
+    "public": "Public",
+    "private": "Private"
+  },
+  "members": {
+    "title": "Member Management",
+    "invite": "Invite Member",
+    "inviteByEmail": "Invite by email",
+    "emailPlaceholder": "Enter email address",
+    "role": "Role",
+    "changeRole": "Change Role",
+    "remove": "Remove Member",
+    "confirmRemove": "Are you sure you want to remove this member?"
+  },
+  "customFields": {
+    "title": "Custom Fields",
+    "add": "Add Field",
+    "edit": "Edit Field",
+    "delete": "Delete Field",
+    "fieldName": "Field Name",
+    "fieldType": "Field Type",
+    "required": "Required",
+    "types": {
+      "text": "Text",
+      "number": "Number",
+      "date": "Date",
+      "select": "Dropdown",
+      "multiSelect": "Multi-select",
+      "checkbox": "Checkbox"
+    }
+  },
+  "notifications": {
+    "title": "Notification Settings",
+    "email": "Email Notifications",
+    "inApp": "In-app Notifications",
+    "taskAssigned": "When a task is assigned to me",
+    "taskCompleted": "When a task is completed",
+    "commentAdded": "When a comment is added",
+    "dueDateApproaching": "When a due date is approaching"
+  },
+  "danger": {
+    "title": "Danger Zone",
+    "archive": "Archive Project",
+    "archiveDescription": "Archive this project. The project will become read-only.",
+    "delete": "Delete Project",
+    "deleteDescription": "Permanently delete this project and all its data. This action cannot be undone.",
+    "confirmArchive": "Are you sure you want to archive this project?",
+    "confirmDelete": "Are you sure you want to delete this project? Type the project name to confirm:",
+    "typeToConfirm": "Type \"{{name}}\" to confirm"
+  },
+  "messages": {
+    "saved": "Settings saved",
+    "memberInvited": "Invitation sent",
+    "memberRemoved": "Member removed",
+    "roleChanged": "Role changed"
+  }
+}

frontend/public/locales/en/spaces.json

@@ -0,0 +1,39 @@
+{
+  "title": "Spaces",
+  "createSpace": "Create Space",
+  "editSpace": "Edit Space",
+  "deleteSpace": "Delete Space",
+  "fields": {
+    "name": "Name",
+    "namePlaceholder": "Enter space name",
+    "description": "Description",
+    "descriptionPlaceholder": "Enter space description",
+    "icon": "Icon",
+    "color": "Color"
+  },
+  "members": {
+    "title": "Members",
+    "add": "Add Member",
+    "remove": "Remove Member",
+    "role": "Role",
+    "owner": "Owner",
+    "admin": "Admin",
+    "member": "Member",
+    "viewer": "Viewer"
+  },
+  "stats": {
+    "projects": "Projects",
+    "members": "Members",
+    "tasks": "Tasks"
+  },
+  "messages": {
+    "created": "Space created",
+    "updated": "Space updated",
+    "deleted": "Space deleted",
+    "confirmDelete": "Are you sure you want to delete this space? This will delete all related projects and tasks."
+  },
+  "empty": {
+    "title": "No Spaces",
+    "description": "Create your first space to organize projects"
+  }
+}

frontend/public/locales/en/tasks.json

@@ -0,0 +1,134 @@
+{
+  "title": "Tasks",
+  "createTask": "Create Task",
+  "editTask": "Edit Task",
+  "deleteTask": "Delete Task",
+  "taskDetails": "Task Details",
+  "fields": {
+    "title": "Title",
+    "titlePlaceholder": "Enter task title",
+    "description": "Description",
+    "descriptionPlaceholder": "Enter task description",
+    "status": "Status",
+    "priority": "Priority",
+    "assignee": "Assignee",
+    "dueDate": "Due Date",
+    "startDate": "Start Date",
+    "estimatedHours": "Estimated Hours",
+    "actualHours": "Actual Hours",
+    "progress": "Progress",
+    "tags": "Tags",
+    "parent": "Parent Task",
+    "subtasks": "Subtasks",
+    "attachments": "Attachments",
+    "comments": "Comments",
+    "watchers": "Watchers",
+    "blockers": "Blockers"
+  },
+  "status": {
+    "todo": "To Do",
+    "in_progress": "In Progress",
+    "review": "In Review",
+    "done": "Done",
+    "cancelled": "Cancelled",
+    "blocked": "Blocked"
+  },
+  "priority": {
+    "low": "Low",
+    "medium": "Medium",
+    "high": "High",
+    "urgent": "Urgent"
+  },
+  "views": {
+    "list": "List",
+    "kanban": "Kanban",
+    "calendar": "Calendar",
+    "gantt": "Gantt",
+    "timeline": "Timeline"
+  },
+  "filters": {
+    "all": "All Tasks",
+    "myTasks": "My Tasks",
+    "unassigned": "Unassigned",
+    "overdue": "Overdue",
+    "dueThisWeek": "Due This Week",
+    "highPriority": "High Priority",
+    "recentlyUpdated": "Recently Updated"
+  },
+  "sort": {
+    "sortBy": "Sort by",
+    "dueDate": "Due Date",
+    "priority": "Priority",
+    "status": "Status",
+    "title": "Title",
+    "createdAt": "Created At",
+    "updatedAt": "Updated At",
+    "ascending": "Ascending",
+    "descending": "Descending"
+  },
+  "actions": {
+    "assign": "Assign",
+    "reassign": "Reassign",
+    "changeStatus": "Change Status",
+    "changePriority": "Change Priority",
+    "addSubtask": "Add Subtask",
+    "addComment": "Add Comment",
+    "addAttachment": "Add Attachment",
+    "addWatcher": "Add Watcher",
+    "removeWatcher": "Remove Watcher",
+    "duplicate": "Duplicate Task",
+    "archive": "Archive",
+    "restore": "Restore",
+    "moveToProject": "Move to Project"
+  },
+  "subtasks": {
+    "title": "Subtasks",
+    "add": "Add Subtask",
+    "placeholder": "Enter subtask title",
+    "completed": "{{count}} / {{total}} completed",
+    "empty": "No subtasks"
+  },
+  "comments": {
+    "title": "Comments",
+    "add": "Add Comment",
+    "placeholder": "Write your comment...",
+    "edited": "edited",
+    "delete": "Delete Comment",
+    "confirmDelete": "Are you sure you want to delete this comment?",
+    "empty": "No comments yet",
+    "reply": "Reply"
+  },
+  "attachments": {
+    "title": "Attachments",
+    "add": "Add Attachment",
+    "upload": "Upload File",
+    "dragDrop": "Drag and drop files here or click to upload",
+    "maxSize": "Max file size: {{size}}MB",
+    "downloading": "Downloading...",
+    "empty": "No attachments"
+  },
+  "blockers": {
+    "title": "Blockers",
+    "add": "Add Blocker",
+    "blockedBy": "Blocked by",
+    "blocking": "Blocking",
+    "remove": "Remove blocker",
+    "empty": "No blockers"
+  },
+  "messages": {
+    "created": "Task created",
+    "updated": "Task updated",
+    "deleted": "Task deleted",
+    "statusChanged": "Status changed to \"{{status}}\"",
+    "assigned": "Assigned to {{assignee}}",
+    "unassigned": "Unassigned",
+    "commentAdded": "Comment added",
+    "attachmentUploaded": "Attachment uploaded",
+    "confirmDelete": "Are you sure you want to delete this task? This action cannot be undone."
+  },
+  "empty": {
+    "title": "No Tasks",
+    "description": "There are no tasks yet. Create your first task to get started!",
+    "filtered": "No tasks match your filters"
+  }
+}

frontend/public/locales/en/workload.json

@@ -0,0 +1,42 @@
+{
+  "title": "Workload",
+  "subtitle": "View team member workload distribution",
+  "filters": {
+    "project": "Project",
+    "allProjects": "All Projects",
+    "dateRange": "Date Range",
+    "thisWeek": "This Week",
+    "thisMonth": "This Month",
+    "custom": "Custom"
+  },
+  "metrics": {
+    "totalHours": "Total Hours",
+    "assignedTasks": "Assigned Tasks",
+    "completedTasks": "Completed Tasks",
+    "overdueTasks": "Overdue Tasks",
+    "utilization": "Utilization"
+  },
+  "chart": {
+    "hoursPerDay": "Hours per Day",
+    "taskDistribution": "Task Distribution",
+    "byProject": "By Project",
+    "byPriority": "By Priority"
+  },
+  "team": {
+    "title": "Team Members",
+    "member": "Member",
+    "allocated": "Allocated",
+    "available": "Available",
+    "overloaded": "Overloaded",
+    "underutilized": "Underutilized"
+  },
+  "status": {
+    "balanced": "Balanced",
+    "overloaded": "Overloaded",
+    "underutilized": "Underutilized"
+  },
+  "empty": {
+    "title": "No Workload Data",
+    "description": "Not enough data to display workload"
+  }
+}