Add login gate

Protect UI with PHP session login and secure APIs.

- Add login/logout pages and session auth helper
- Serve protected content from index.php
- Redirect index.html to index.php to prevent bypass
- Require auth for image list/upload APIs

Co-authored-by: Cursor <cursoragent@cursor.com>

logout.php

@@ -0,0 +1,24 @@
+<?php
+declare(strict_types=1);
+require_once __DIR__ . '/auth/auth.php';
+
+dreamgirl_session_start();
+$_SESSION = [];
+
+if (ini_get('session.use_cookies')) {
+  $params = session_get_cookie_params();
+  setcookie(
+    session_name(),
+    '',
+    time() - 42000,
+    $params['path'] ?? '/',
+    $params['domain'] ?? '',
+    (bool)($params['secure'] ?? false),
+    (bool)($params['httponly'] ?? true)
+  );
+}
+
+session_destroy();
+header('Location: /login.php');
+exit;
+