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>

api/upload_image.php

@@ -1,5 +1,7 @@
 <?php
 // Upload handler: saves an uploaded image into /img and returns JSON.
+require_once __DIR__ . '/../auth/auth.php';
+dreamgirl_require_login_json();
 header('Content-Type: application/json; charset=utf-8');
 
 // Limit: 10MB