Fix download modal: use per-backup modals for CSP/Turbo compatibility

- Replace shared modal + inline JS with per-backup modals that have
  filename pre-set in hidden fields (no JavaScript needed)
- Add data-turbo="false" to download forms for native browser handling
- Add data-bs-dismiss="modal" to submit button to auto-close modal
- Add hidden username field for Chrome accessibility best practice
- Fix test: GET on POST-only route returns 404 not 405

tests/Controller/UpdateManagerControllerTest.php

@@ -251,10 +251,10 @@ final class UpdateManagerControllerTest extends WebTestCase
         $client = static::createClient();
         $this->loginAsAdmin($client);
 
-        // GET should return 405 Method Not Allowed
+        // GET returns 404 since no GET route exists for this path
         $client->request('GET', '/en/system/update-manager/backup/download');
 
-        $this->assertResponseStatusCodeSame(405);
+        $this->assertResponseStatusCodeSame(404);
     }
 
     public function testDownloadBackupRequiresAuth(): void