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/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"
+  }
+}