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/workload.json

@@ -0,0 +1,42 @@
+{
+  "title": "Workload",
+  "subtitle": "View team member workload distribution",
+  "filters": {
+    "project": "Project",
+    "allProjects": "All Projects",
+    "dateRange": "Date Range",
+    "thisWeek": "This Week",
+    "thisMonth": "This Month",
+    "custom": "Custom"
+  },
+  "metrics": {
+    "totalHours": "Total Hours",
+    "assignedTasks": "Assigned Tasks",
+    "completedTasks": "Completed Tasks",
+    "overdueTasks": "Overdue Tasks",
+    "utilization": "Utilization"
+  },
+  "chart": {
+    "hoursPerDay": "Hours per Day",
+    "taskDistribution": "Task Distribution",
+    "byProject": "By Project",
+    "byPriority": "By Priority"
+  },
+  "team": {
+    "title": "Team Members",
+    "member": "Member",
+    "allocated": "Allocated",
+    "available": "Available",
+    "overloaded": "Overloaded",
+    "underutilized": "Underutilized"
+  },
+  "status": {
+    "balanced": "Balanced",
+    "overloaded": "Overloaded",
+    "underutilized": "Underutilized"
+  },
+  "empty": {
+    "title": "No Workload Data",
+    "description": "Not enough data to display workload"
+  }
+}