Show server validation errors as individual flash alerts, not one blob

Each missing field shows as its own red error banner at the top of the page
instead of one long string with invisible newlines.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Modules/Members/Controllers/RetroactiveWizardController.php

@@ -49,8 +49,16 @@ class RetroactiveWizardController extends Controller
         $result = RetroactiveMembershipService::createRetroactiveMember($data);
 
         if (!$result['success']) {
-            return $this->redirect('/members/retroactive-wizard')
-                ->withError('فشل إنشاء العضوية: ' . ($result['error'] ?? 'خطأ غير معروف'));
+            $errorMsg = $result['error'] ?? 'خطأ غير معروف';
+            $errorLines = array_filter(explode("\n", $errorMsg));
+            $session = App::getInstance()->session();
+            $alerts = [];
+            foreach ($errorLines as $line) {
+                $alerts[] = ['type' => 'error', 'message' => trim($line)];
+            }
+            $session->flash('_alerts', $alerts);
+            $session->flash('_old_input', $_POST);
+            return $this->redirect('/members/retroactive-wizard');
         }
 
         $memberId = $result['data']['member_id'];