fix: repair form submission with queued files + add comprehensive debug logging

- Replace fetch(redirect:manual) with XMLHttpRequest in file-upload-queue.js.
  The previous fetch-based redirect detection was broken because opaque
  redirects hide the Location header. XHR's responseURL reliably exposes
  the final URL after server-side redirects.

- Add console.log tracing at every decision point in submit interception:
  entry, hasFiles check, enctype check, double-submit guard, XHR status,
  redirect detection, error fallback.

- Add error_log entry-point logging to all 16 admin action files plus
  the partage/index.php submission handler and password gate. Each logs:
  request method, content type/length, POST keys, file counts, and
  queue-specific file counts where applicable.

- Add double-submit guard (_xamxamActiveSubmit) to prevent duplicate
  XHR sends when the native submit handler fires after interception.

app/public/admin/actions/formulaire.php

@@ -3,6 +3,8 @@
 require_once __DIR__ . '/../../../bootstrap.php';
 require_once __DIR__ . '/../../../src/AdminAuth.php';
 
+error_log('[formulaire.php] ENTRY | method=' . $_SERVER['REQUEST_METHOD'] . ' | content_type=' . ($_SERVER['CONTENT_TYPE'] ?? 'none') . ' | content_length=' . ($_SERVER['CONTENT_LENGTH'] ?? '0') . ' | post_keys=' . implode(',', array_keys($_POST)) . ' | files_count=' . count($_FILES) . ' | files_keys=' . implode(',', array_keys($_FILES)) . ' | queue_tfe=' . (isset($_FILES['queue_file']['name']['tfe']) ? count(array_filter($_FILES['queue_file']['name']['tfe'])) : 0));
+
 // Only suppress display_errors in production (cli-server = dev mode).
 if (php_sapi_name() !== 'cli-server') {
     ini_set('display_errors', 0);
@@ -23,7 +25,7 @@ if (!isset($_POST['csrf_token'], $_SESSION['csrf_token'])
     die('Erreur de sécurité : token invalide. Veuillez recharger le formulaire.');
 }
 
-error_log('FILES array: ' . print_r($_FILES, true));
+error_log('[formulaire.php] full FILES array: ' . print_r($_FILES, true));
 
 require_once APP_ROOT . '/src/Controllers/ThesisCreateController.php';
 require_once APP_ROOT . '/src/AppLogger.php';