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/audit.json

@@ -0,0 +1,56 @@
+{
+  "title": "稽核日誌",
+  "subtitle": "追蹤系統中的所有操作記錄",
+  "filters": {
+    "action": "操作類型",
+    "user": "使用者",
+    "entity": "實體類型",
+    "dateRange": "日期範圍",
+    "allActions": "所有操作",
+    "allUsers": "所有使用者",
+    "allEntities": "所有實體"
+  },
+  "actions": {
+    "create": "建立",
+    "update": "更新",
+    "delete": "刪除",
+    "login": "登入",
+    "logout": "登出",
+    "assign": "指派",
+    "unassign": "取消指派",
+    "statusChange": "狀態變更",
+    "comment": "留言",
+    "upload": "上傳",
+    "download": "下載"
+  },
+  "entities": {
+    "task": "任務",
+    "project": "專案",
+    "space": "工作空間",
+    "user": "使用者",
+    "comment": "留言",
+    "attachment": "附件"
+  },
+  "columns": {
+    "timestamp": "時間",
+    "user": "使用者",
+    "action": "操作",
+    "entity": "實體",
+    "details": "詳情",
+    "ipAddress": "IP 位址"
+  },
+  "details": {
+    "before": "變更前",
+    "after": "變更後",
+    "changes": "變更內容"
+  },
+  "export": {
+    "title": "匯出日誌",
+    "csv": "匯出為 CSV",
+    "json": "匯出為 JSON"
+  },
+  "empty": {
+    "title": "沒有稽核記錄",
+    "description": "目前沒有符合條件的稽核記錄"
+  }
+}