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/en/auth.json

@@ -0,0 +1,28 @@
+{
+  "login": {
+    "title": "Login",
+    "subtitle": "Sign in to your account",
+    "email": "Email",
+    "emailPlaceholder": "Enter your email",
+    "password": "Password",
+    "passwordPlaceholder": "Enter your password",
+    "rememberMe": "Remember me",
+    "forgotPassword": "Forgot password?",
+    "submit": "Sign in",
+    "loggingIn": "Signing in...",
+    "noAccount": "Don't have an account?",
+    "signUp": "Sign up"
+  },
+  "errors": {
+    "invalidCredentials": "Invalid email or password",
+    "accountLocked": "Account is locked. Please contact administrator.",
+    "emailRequired": "Email is required",
+    "passwordRequired": "Password is required",
+    "invalidEmail": "Please enter a valid email address",
+    "loginFailed": "Login failed. Please try again later."
+  },
+  "welcome": {
+    "title": "Project Control Center",
+    "subtitle": "Manage your projects, tasks, and teams"
+  }
+}