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/health.json

@@ -0,0 +1,48 @@
+{
+  "title": "Project Health",
+  "subtitle": "Monitor the overall health of projects",
+  "overall": {
+    "title": "Overall Health",
+    "healthy": "Healthy",
+    "atRisk": "At Risk",
+    "critical": "Critical"
+  },
+  "metrics": {
+    "schedule": "Schedule",
+    "budget": "Budget",
+    "scope": "Scope",
+    "quality": "Quality",
+    "resources": "Resources"
+  },
+  "status": {
+    "onTrack": "On Track",
+    "delayed": "Delayed",
+    "ahead": "Ahead",
+    "overBudget": "Over Budget",
+    "underBudget": "Under Budget"
+  },
+  "indicators": {
+    "title": "Health Indicators",
+    "taskCompletion": "Task Completion Rate",
+    "onTimeDelivery": "On-time Delivery Rate",
+    "blockedTasks": "Blocked Tasks",
+    "overdueRate": "Overdue Rate",
+    "velocityTrend": "Velocity Trend"
+  },
+  "risks": {
+    "title": "Risks",
+    "high": "High Risk",
+    "medium": "Medium Risk",
+    "low": "Low Risk",
+    "mitigated": "Mitigated"
+  },
+  "actions": {
+    "viewDetails": "View Details",
+    "exportReport": "Export Report",
+    "setAlert": "Set Alert"
+  },
+  "empty": {
+    "title": "No Health Data",
+    "description": "The project needs more data to display health metrics"
+  }
+}