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/package.json

@@ -19,8 +19,12 @@
     "@fullcalendar/timegrid": "^6.1.20",
     "axios": "^1.6.0",
     "frappe-gantt": "^1.0.4",
+    "i18next": "^25.7.4",
+    "i18next-browser-languagedetector": "^8.2.0",
+    "i18next-http-backend": "^3.0.2",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
+    "react-i18next": "^16.5.1",
     "react-router-dom": "^6.21.0"
   },
   "devDependencies": {