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/visibility.php

@@ -1,6 +1,7 @@
 <?php
 require_once __DIR__ . '/../../../bootstrap.php';
 require_once __DIR__ . '/../../../src/AdminAuth.php';
+error_log('[visibility.php] ENTRY | method=' . $_SERVER['REQUEST_METHOD'] . ' | action=' . ($_POST['action'] ?? 'none') . ' | is_bulk=' . (!empty($_POST['bulk']) ? '1' : '0') . ' | post_keys=' . implode(',', array_keys($_POST)));
 AdminAuth::requireLogin();
 
 if (!isset($_POST['csrf_token'], $_SESSION['csrf_token'])