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/workload.json

@@ -0,0 +1,42 @@
+{
+  "title": "工作負載",
+  "subtitle": "檢視團隊成員的工作負載分佈",
+  "filters": {
+    "project": "專案",
+    "allProjects": "所有專案",
+    "dateRange": "日期範圍",
+    "thisWeek": "本週",
+    "thisMonth": "本月",
+    "custom": "自訂"
+  },
+  "metrics": {
+    "totalHours": "總工時",
+    "assignedTasks": "指派任務數",
+    "completedTasks": "已完成任務",
+    "overdueTasks": "逾期任務",
+    "utilization": "使用率"
+  },
+  "chart": {
+    "hoursPerDay": "每日工時",
+    "taskDistribution": "任務分佈",
+    "byProject": "依專案",
+    "byPriority": "依優先順序"
+  },
+  "team": {
+    "title": "團隊成員",
+    "member": "成員",
+    "allocated": "已分配",
+    "available": "可用",
+    "overloaded": "超載",
+    "underutilized": "低使用率"
+  },
+  "status": {
+    "balanced": "平衡",
+    "overloaded": "超載",
+    "underutilized": "低使用率"
+  },
+  "empty": {
+    "title": "沒有工作負載資料",
+    "description": "目前沒有足夠的資料來顯示工作負載"
+  }
+}