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"
+  }
+}