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/spaces.json

@@ -0,0 +1,39 @@
+{
+  "title": "Spaces",
+  "createSpace": "Create Space",
+  "editSpace": "Edit Space",
+  "deleteSpace": "Delete Space",
+  "fields": {
+    "name": "Name",
+    "namePlaceholder": "Enter space name",
+    "description": "Description",
+    "descriptionPlaceholder": "Enter space description",
+    "icon": "Icon",
+    "color": "Color"
+  },
+  "members": {
+    "title": "Members",
+    "add": "Add Member",
+    "remove": "Remove Member",
+    "role": "Role",
+    "owner": "Owner",
+    "admin": "Admin",
+    "member": "Member",
+    "viewer": "Viewer"
+  },
+  "stats": {
+    "projects": "Projects",
+    "members": "Members",
+    "tasks": "Tasks"
+  },
+  "messages": {
+    "created": "Space created",
+    "updated": "Space updated",
+    "deleted": "Space deleted",
+    "confirmDelete": "Are you sure you want to delete this space? This will delete all related projects and tasks."
+  },
+  "empty": {
+    "title": "No Spaces",
+    "description": "Create your first space to organize projects"
+  }
+}