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/partage/index.php

@@ -93,6 +93,7 @@ App::boot();
 
 // ── POST: form submission ─────────────────────────────────────────────────────
 if ($_SERVER['REQUEST_METHOD'] === 'POST' && $action === 'submit') {
+    error_log('[partage/index.php] SUBMIT ENTRY | slug=' . $slug . ' | content_type=' . ($_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));
     handleShareLinkSubmission($slug);
     exit;
 }
@@ -187,6 +188,7 @@ function renderShareLinkError(string $title, string $message): void
 function requirePasswordGate(array $link, string $slug): void
 {
     if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['share_password'])) {
+        error_log('[partage/password-gate] ENTRY | slug=' . $slug . ' | post_keys=' . implode(',', array_keys($_POST)));
         require_once APP_ROOT . '/src/ShareLink.php';
         $shareLinkModel = new ShareLink(Database::getInstance());