feat(admin): Add group filtering for account listings

- Add groupID parameter to ListAccounts and ListWithFilters methods
- Implement account filtering by group ID in repository query
- Add group query parameter parsing in account handler
- Update all ListAccounts/ListWithFilters call sites with groupID parameter
- Add group filter UI component to AccountTableFilters
- Add i18n translations for group filter label in English and Chinese
- Update API contract and test stubs to reflect new signature
- Enable filtering accounts by their assigned groups in admin panel

backend/internal/service/admin_service_search_test.go

@@ -24,7 +24,7 @@ type accountRepoStubForAdminList struct {
 	listWithFiltersErr      error
 }
 
-func (s *accountRepoStubForAdminList) ListWithFilters(_ context.Context, params pagination.PaginationParams, platform, accountType, status, search string) ([]Account, *pagination.PaginationResult, error) {
+func (s *accountRepoStubForAdminList) ListWithFilters(_ context.Context, params pagination.PaginationParams, platform, accountType, status, search string, groupID int64) ([]Account, *pagination.PaginationResult, error) {
 	s.listWithFiltersCalls++
 	s.listWithFiltersParams = params
 	s.listWithFiltersPlatform = platform
@@ -168,7 +168,7 @@ func TestAdminService_ListAccounts_WithSearch(t *testing.T) {
 		}
 		svc := &adminServiceImpl{accountRepo: repo}
 
-		accounts, total, err := svc.ListAccounts(context.Background(), 1, 20, PlatformGemini, AccountTypeOAuth, StatusActive, "acc")
+		accounts, total, err := svc.ListAccounts(context.Background(), 1, 20, PlatformGemini, AccountTypeOAuth, StatusActive, "acc", 0)
 		require.NoError(t, err)
 		require.Equal(t, int64(10), total)
 		require.Equal(t, []Account{{ID: 1, Name: "acc"}}, accounts)