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/health.json

@@ -0,0 +1,48 @@
+{
+  "title": "專案健康度",
+  "subtitle": "監控專案的整體健康狀況",
+  "overall": {
+    "title": "整體健康度",
+    "healthy": "健康",
+    "atRisk": "風險中",
+    "critical": "危急"
+  },
+  "metrics": {
+    "schedule": "進度",
+    "budget": "預算",
+    "scope": "範圍",
+    "quality": "品質",
+    "resources": "資源"
+  },
+  "status": {
+    "onTrack": "正常進行",
+    "delayed": "延遲",
+    "ahead": "超前",
+    "overBudget": "超支",
+    "underBudget": "低於預算"
+  },
+  "indicators": {
+    "title": "健康指標",
+    "taskCompletion": "任務完成率",
+    "onTimeDelivery": "準時交付率",
+    "blockedTasks": "阻擋任務數",
+    "overdueRate": "逾期率",
+    "velocityTrend": "速度趨勢"
+  },
+  "risks": {
+    "title": "風險",
+    "high": "高風險",
+    "medium": "中風險",
+    "low": "低風險",
+    "mitigated": "已緩解"
+  },
+  "actions": {
+    "viewDetails": "查看詳情",
+    "exportReport": "匯出報告",
+    "setAlert": "設定警示"
+  },
+  "empty": {
+    "title": "沒有健康度資料",
+    "description": "專案需要更多資料才能顯示健康度指標"
+  }
+}