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/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"
+  }
+}