SEO overhaul: product schema, dynamic sitemap, favicon, og-image fix

- product.php: set metaTitle, metaDescription, canonicalUrl, ogImage,
  ogType=product, productSchema (JSON-LD with price/availability/reviews),
  and breadcrumbs variables for header.php to consume
- sitemap.php: dynamic XML sitemap generated from DB — includes all 30
  active products + static pages; robots.txt now points here
- header.php: fix favicon links (favicon.ico in root + icon-192.png);
  fix productSchema output (was double-encoding via json_encode)
- robots.txt: point Sitemap directive to /sitemap.php

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/header.php

@@ -54,7 +54,7 @@ $customerUser = $isLoggedIn ? CustomerAuth::getUser() : null;
     </script>
     <?php endif; ?>
     <?php if (!empty($productSchema)): ?>
-    <script type="application/ld+json"><?= json_encode($productSchema,JSON_UNESCAPED_SLASHES) ?></script>
+    <script type="application/ld+json"><?= $productSchema ?></script>
     <?php endif; ?>
 
     <!-- PWA Meta Tags -->
@@ -78,8 +78,10 @@ $customerUser = $isLoggedIn ? CustomerAuth::getUser() : null;
     <link rel="stylesheet" href="/assets/css/style.css?v=<?= filemtime($_SERVER['DOCUMENT_ROOT'] . '/assets/css/style.css') ?>">
     
     <!-- Favicon -->
-    <link rel="icon" href="/assets/images/logo.png" type="image/png">
-    
+    <link rel="icon" type="image/x-icon" href="/favicon.ico">
+    <link rel="icon" type="image/png" sizes="192x192" href="/assets/images/icon-192.png">
+    <link rel="apple-touch-icon" sizes="192x192" href="/assets/images/icon-192.png">
+
     <?php if (strpos($_SERVER['REQUEST_URI'], 'payment') !== false || strpos($_SERVER['REQUEST_URI'], 'checkout') !== false): ?>
     <script src="https://js.stripe.com/v3/"></script>
     <?php endif; ?>