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

backend/internal/service/gemini_multiplatform_test.go

@@ -226,6 +226,10 @@ func (m *mockGroupRepoForGemini) GetAccountIDsByGroupIDs(ctx context.Context, gr
 	return nil, nil
 }
 
+func (m *mockGroupRepoForGemini) UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return nil
+}
+
 var _ GroupRepository = (*mockGroupRepoForGemini)(nil)
 
 // mockGatewayCacheForGemini Gemini 测试用的 cache mock