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/settings.json

@@ -0,0 +1,73 @@
+{
+  "title": "設定",
+  "projectSettings": "專案設定",
+  "tabs": {
+    "general": "一般",
+    "members": "成員",
+    "customFields": "自訂欄位",
+    "notifications": "通知",
+    "integrations": "整合",
+    "danger": "危險區域"
+  },
+  "general": {
+    "title": "一般設定",
+    "projectName": "專案名稱",
+    "description": "描述",
+    "status": "狀態",
+    "visibility": "可見性",
+    "public": "公開",
+    "private": "私人"
+  },
+  "members": {
+    "title": "成員管理",
+    "invite": "邀請成員",
+    "inviteByEmail": "透過電子郵件邀請",
+    "emailPlaceholder": "輸入電子郵件地址",
+    "role": "角色",
+    "changeRole": "變更角色",
+    "remove": "移除成員",
+    "confirmRemove": "確定要移除此成員嗎？"
+  },
+  "customFields": {
+    "title": "自訂欄位",
+    "add": "新增欄位",
+    "edit": "編輯欄位",
+    "delete": "刪除欄位",
+    "fieldName": "欄位名稱",
+    "fieldType": "欄位類型",
+    "required": "必填",
+    "types": {
+      "text": "文字",
+      "number": "數字",
+      "date": "日期",
+      "select": "下拉選單",
+      "multiSelect": "多選",
+      "checkbox": "核取方塊"
+    }
+  },
+  "notifications": {
+    "title": "通知設定",
+    "email": "電子郵件通知",
+    "inApp": "應用內通知",
+    "taskAssigned": "任務指派給我時",
+    "taskCompleted": "任務完成時",
+    "commentAdded": "新增留言時",
+    "dueDateApproaching": "截止日期即將到來時"
+  },
+  "danger": {
+    "title": "危險區域",
+    "archive": "封存專案",
+    "archiveDescription": "封存此專案。封存後專案將變為唯讀。",
+    "delete": "刪除專案",
+    "deleteDescription": "永久刪除此專案及其所有資料。此操作無法復原。",
+    "confirmArchive": "確定要封存此專案嗎？",
+    "confirmDelete": "確定要刪除此專案嗎？請輸入專案名稱以確認：",
+    "typeToConfirm": "輸入「{{name}}」以確認"
+  },
+  "messages": {
+    "saved": "設定已儲存",
+    "memberInvited": "已發送邀請",
+    "memberRemoved": "成員已移除",
+    "roleChanged": "角色已變更"
+  }
+}