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/index.php

@@ -17,6 +17,7 @@ $importResults = [];
 $importDone    = false;
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
+    error_log('[admin/index.php] IMPORT ENTRY | method=' . $_SERVER['REQUEST_METHOD'] . ' | content_type=' . ($_SERVER['CONTENT_TYPE'] ?? 'none') . ' | csv_file_error=' . ($_FILES['csv_file']['error'] ?? 'none') . ' | post_keys=' . implode(',', array_keys($_POST)));
     if (!isset($_POST['csrf_token']) || !hash_equals($_SESSION['csrf_token'], $_POST['csrf_token'])) {
         $importErrors[] = "Erreur de sécurité : token invalide.";
     } else {