feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

frontend/package.json

@@ -27,6 +27,7 @@
     "qrcode": "^1.5.4",
     "vue": "^3.4.0",
     "vue-chartjs": "^5.3.0",
+    "vue-draggable-plus": "^0.6.1",
     "vue-i18n": "^9.14.5",
     "vue-router": "^4.2.5",
     "xlsx": "^0.18.5"