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/account_service_delete_test.go

@@ -75,7 +75,7 @@ func (s *accountRepoStub) List(ctx context.Context, params pagination.Pagination
 	panic("unexpected List call")
 }
 
-func (s *accountRepoStub) ListWithFilters(ctx context.Context, params pagination.PaginationParams, platform, accountType, status, search string) ([]Account, *pagination.PaginationResult, error) {
+func (s *accountRepoStub) ListWithFilters(ctx context.Context, params pagination.PaginationParams, platform, accountType, status, search string, groupID int64) ([]Account, *pagination.PaginationResult, error) {
 	panic("unexpected ListWithFilters call")
 }