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/zh-TW/dashboard.json

@@ -0,0 +1,47 @@
+{
+  "title": "儀表板",
+  "welcome": "歡迎回來，{{name}}",
+  "stats": {
+    "myTasks": "我的任務",
+    "overdueTasks": "逾期任務",
+    "completedTasks": "已完成任務",
+    "activeProjects": "進行中專案",
+    "teamMembers": "團隊成員",
+    "totalTasks": "總任務數",
+    "pendingReview": "待審核"
+  },
+  "sections": {
+    "recentActivity": "近期活動",
+    "upcomingDeadlines": "即將到期",
+    "myAssignedTasks": "指派給我的任務",
+    "projectOverview": "專案概覽",
+    "quickActions": "快速操作"
+  },
+  "activity": {
+    "taskCreated": "建立了任務「{{task}}」",
+    "taskCompleted": "完成了任務「{{task}}」",
+    "taskAssigned": "將任務「{{task}}」指派給 {{assignee}}",
+    "commentAdded": "在任務「{{task}}」新增了留言",
+    "projectCreated": "建立了專案「{{project}}」",
+    "noRecentActivity": "暫無近期活動"
+  },
+  "deadlines": {
+    "today": "今天到期",
+    "tomorrow": "明天到期",
+    "thisWeek": "本週到期",
+    "overdue": "已逾期",
+    "noUpcoming": "近期沒有待處理的截止日期"
+  },
+  "quickActions": {
+    "createTask": "建立任務",
+    "createProject": "建立專案",
+    "viewAllTasks": "查看所有任務",
+    "viewWorkload": "查看工作負載"
+  },
+  "widgets": {
+    "tasksByStatus": "依狀態分類的任務",
+    "tasksByPriority": "依優先順序分類的任務",
+    "projectProgress": "專案進度",
+    "teamWorkload": "團隊工作負載"
+  }
+}