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/auth.json

@@ -0,0 +1,28 @@
+{
+  "login": {
+    "title": "登入",
+    "subtitle": "登入您的帳戶",
+    "email": "電子郵件",
+    "emailPlaceholder": "請輸入電子郵件",
+    "password": "密碼",
+    "passwordPlaceholder": "請輸入密碼",
+    "rememberMe": "記住我",
+    "forgotPassword": "忘記密碼？",
+    "submit": "登入",
+    "loggingIn": "登入中...",
+    "noAccount": "還沒有帳戶？",
+    "signUp": "註冊"
+  },
+  "errors": {
+    "invalidCredentials": "電子郵件或密碼錯誤",
+    "accountLocked": "帳戶已被鎖定，請聯繫管理員",
+    "emailRequired": "請輸入電子郵件",
+    "passwordRequired": "請輸入密碼",
+    "invalidEmail": "請輸入有效的電子郵件地址",
+    "loginFailed": "登入失敗，請稍後再試"
+  },
+  "welcome": {
+    "title": "專案控制中心",
+    "subtitle": "管理您的專案、任務和團隊"
+  }
+}