feat(groups): add rate multipliers management modal

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints

backend/internal/handler/admin/admin_service_stub_test.go

@@ -175,6 +175,18 @@ func (s *stubAdminService) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return s.apiKeys, int64(len(s.apiKeys)), nil
 }
 
+func (s *stubAdminService) GetGroupRateMultipliers(_ context.Context, _ int64) ([]service.UserGroupRateEntry, error) {
+	return nil, nil
+}
+
+func (s *stubAdminService) ClearGroupRateMultipliers(_ context.Context, _ int64) error {
+	return nil
+}
+
+func (s *stubAdminService) BatchSetGroupRateMultipliers(_ context.Context, _ int64, _ []service.GroupRateMultiplierInput) error {
+	return nil
+}
+
 func (s *stubAdminService) ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string, groupID int64) ([]service.Account, int64, error) {
 	return s.accounts, int64(len(s.accounts)), nil
 }