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/projects.json

@@ -0,0 +1,50 @@
+{
+  "title": "專案",
+  "createProject": "建立專案",
+  "editProject": "編輯專案",
+  "deleteProject": "刪除專案",
+  "projectSettings": "專案設定",
+  "fields": {
+    "name": "名稱",
+    "namePlaceholder": "輸入專案名稱",
+    "description": "描述",
+    "descriptionPlaceholder": "輸入專案描述",
+    "status": "狀態",
+    "startDate": "開始日期",
+    "endDate": "結束日期",
+    "owner": "專案負責人",
+    "space": "工作空間"
+  },
+  "status": {
+    "planning": "規劃中",
+    "active": "進行中",
+    "on_hold": "暫停",
+    "completed": "已完成",
+    "cancelled": "已取消"
+  },
+  "tabs": {
+    "overview": "概覽",
+    "tasks": "任務",
+    "members": "成員",
+    "settings": "設定",
+    "files": "檔案",
+    "activity": "活動紀錄"
+  },
+  "stats": {
+    "totalTasks": "總任務數",
+    "completedTasks": "已完成",
+    "inProgress": "進行中",
+    "overdue": "逾期",
+    "progress": "整體進度"
+  },
+  "messages": {
+    "created": "專案已建立",
+    "updated": "專案已更新",
+    "deleted": "專案已刪除",
+    "confirmDelete": "確定要刪除此專案嗎？此操作將刪除所有相關任務。"
+  },
+  "empty": {
+    "title": "沒有專案",
+    "description": "建立您的第一個專案來開始管理任務"
+  }
+}