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/login.php

@@ -2,6 +2,8 @@
 require_once __DIR__ . '/../../bootstrap.php';
 require_once __DIR__ . '/../../src/AdminAuth.php';
 
+error_log('[admin/login.php] ENTRY | method=' . $_SERVER['REQUEST_METHOD'] . ' | is_auth=' . (AdminAuth::isAuthenticated() ? '1' : '0') . ' | has_password=' . (AdminAuth::hasPassword() ? '1' : '0'));
+
 if (!AdminAuth::hasPassword()) {
     header('Location: /admin/');
     exit;