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/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"
+  }
+}