Add bulk transfer functionality for domains and groups

Introduces bulk transfer for domains and groups, available only to admins. Refactors admin checks to use isAdmin(), adds user selection for transfers, and updates views to include transfer buttons and modals for domain transfers.

app/Controllers/DomainController.php

@@ -64,10 +64,18 @@ class DomainController extends Controller
         // Format domains for display
         $formattedDomains = \App\Helpers\DomainHelper::formatMultiple($result['domains']);
 
+        // Get users for transfer functionality (admin only)
+        $users = [];
+        if (\Core\Auth::isAdmin()) {
+            $userModel = new \App\Models\User();
+            $users = $userModel->all();
+        }
+
         $this->view('domains/index', [
             'domains' => $formattedDomains,
             'groups' => $groups,
             'allTags' => $allTags,
+            'users' => $users,
             'filters' => [
                 'search' => $search,
                 'status' => $status,