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/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"
+  }
+}