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/common.json

@@ -0,0 +1,102 @@
+{
+  "buttons": {
+    "save": "儲存",
+    "cancel": "取消",
+    "delete": "刪除",
+    "edit": "編輯",
+    "create": "建立",
+    "close": "關閉",
+    "confirm": "確認",
+    "back": "返回",
+    "next": "下一步",
+    "previous": "上一步",
+    "submit": "提交",
+    "reset": "重置",
+    "search": "搜尋",
+    "filter": "篩選",
+    "export": "匯出",
+    "import": "匯入",
+    "refresh": "重新整理",
+    "add": "新增",
+    "remove": "移除",
+    "view": "檢視",
+    "download": "下載",
+    "upload": "上傳"
+  },
+  "labels": {
+    "loading": "載入中...",
+    "noData": "暫無資料",
+    "required": "必填",
+    "optional": "選填",
+    "all": "全部",
+    "none": "無",
+    "yes": "是",
+    "no": "否",
+    "active": "啟用",
+    "inactive": "停用",
+    "enabled": "已啟用",
+    "disabled": "已停用",
+    "actions": "操作",
+    "details": "詳情",
+    "description": "描述",
+    "name": "名稱",
+    "title": "標題",
+    "status": "狀態",
+    "type": "類型",
+    "date": "日期",
+    "time": "時間",
+    "createdAt": "建立時間",
+    "updatedAt": "更新時間",
+    "createdBy": "建立者",
+    "assignee": "負責人",
+    "selectAssignee": "選擇負責人...",
+    "searchUsers": "搜尋使用者...",
+    "noUsersFound": "找不到使用者",
+    "typeToSearch": "輸入以搜尋使用者"
+  },
+  "messages": {
+    "success": "操作成功",
+    "error": "操作失敗",
+    "confirmDelete": "確定要刪除嗎？此操作無法復原。",
+    "unsavedChanges": "您有未儲存的變更，確定要離開嗎？",
+    "networkError": "網路連線錯誤，請稍後再試",
+    "sessionExpired": "工作階段已過期，請重新登入",
+    "permissionDenied": "您沒有權限執行此操作",
+    "notFound": "找不到請求的資源"
+  },
+  "validation": {
+    "required": "此欄位為必填",
+    "email": "請輸入有效的電子郵件地址",
+    "minLength": "至少需要 {{min}} 個字元",
+    "maxLength": "最多 {{max}} 個字元",
+    "invalidFormat": "格式不正確"
+  },
+  "nav": {
+    "dashboard": "儀表板",
+    "spaces": "工作空間",
+    "projects": "專案",
+    "tasks": "任務",
+    "workload": "工作負載",
+    "health": "專案健康度",
+    "audit": "稽核日誌",
+    "settings": "設定",
+    "logout": "登出"
+  },
+  "language": {
+    "switch": "切換語言",
+    "zhTW": "繁體中文",
+    "en": "English"
+  },
+  "notifications": {
+    "title": "通知",
+    "markAllRead": "全部標為已讀",
+    "noNotifications": "沒有通知",
+    "viewAll": "查看全部"
+  },
+  "pagination": {
+    "page": "第 {{page}} 頁",
+    "of": "共 {{total}} 頁",
+    "showing": "顯示 {{from}}-{{to}} 筆，共 {{total}} 筆",
+    "itemsPerPage": "每頁顯示"
+  }
+}