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/spaces.json

@@ -0,0 +1,39 @@
+{
+  "title": "工作空間",
+  "createSpace": "建立工作空間",
+  "editSpace": "編輯工作空間",
+  "deleteSpace": "刪除工作空間",
+  "fields": {
+    "name": "名稱",
+    "namePlaceholder": "輸入工作空間名稱",
+    "description": "描述",
+    "descriptionPlaceholder": "輸入工作空間描述",
+    "icon": "圖示",
+    "color": "顏色"
+  },
+  "members": {
+    "title": "成員",
+    "add": "新增成員",
+    "remove": "移除成員",
+    "role": "角色",
+    "owner": "擁有者",
+    "admin": "管理員",
+    "member": "成員",
+    "viewer": "檢視者"
+  },
+  "stats": {
+    "projects": "專案數",
+    "members": "成員數",
+    "tasks": "任務數"
+  },
+  "messages": {
+    "created": "工作空間已建立",
+    "updated": "工作空間已更新",
+    "deleted": "工作空間已刪除",
+    "confirmDelete": "確定要刪除此工作空間嗎？此操作將刪除所有相關專案和任務。"
+  },
+  "empty": {
+    "title": "沒有工作空間",
+    "description": "建立您的第一個工作空間來組織專案"
+  }
+}