feat: Add CSRF protection to user preferences API

- Add CSRF validation to user_preferences.php
- Protects POST and DELETE methods
- Completes CSRF protection for all API endpoints

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

api/user_preferences.php

@@ -17,6 +17,17 @@ if (!isset($_SESSION['user']) || !isset($_SESSION['user']['user_id'])) {
     exit;
 }
 
+// CSRF Protection
+require_once dirname(__DIR__) . '/middleware/CsrfMiddleware.php';
+if ($_SERVER['REQUEST_METHOD'] === 'POST' || $_SERVER['REQUEST_METHOD'] === 'DELETE') {
+    $csrfToken = $_SERVER['HTTP_X_CSRF_TOKEN'] ?? '';
+    if (!CsrfMiddleware::validateToken($csrfToken)) {
+        http_response_code(403);
+        echo json_encode(['success' => false, 'error' => 'Invalid CSRF token']);
+        exit;
+    }
+}
+
 $userId = $_SESSION['user']['user_id'];
 
 // Create database connection