Fix boot safely, add social OG image, restore performance gains

Guard fitCanvas until Engine is ready, show a boot placeholder during shader compile, generate a 1200x630 OG image for X and WhatsApp, and point repo meta to lumenshaders.vercel.app.

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

docs.html

@@ -7,7 +7,10 @@
 <meta name="description" content="How to use LUMEN: styles, synth styles, color, motion, share codes, gradient sets and exporting PNG, video or GIF." />
 <link rel="canonical" href="https://lumenshaders.vercel.app/docs.html" />
 <meta property="og:url" content="https://lumenshaders.vercel.app/docs.html" />
-<meta property="og:image" content="https://lumenshaders.vercel.app/assets/og.png" />
+<meta property="og:image" content="https://lumenshaders.vercel.app/assets/og.jpg" />
+<meta property="og:image:type" content="image/jpeg" />
+<meta property="og:image:width" content="1200" />
+<meta property="og:image:height" content="630" />
 <link rel="icon" type="image/png" sizes="64x64" href="assets/favicon.png" />
 <link rel="preconnect" href="https://fonts.googleapis.com" />
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
@@ -173,5 +176,9 @@
   </div>
 </div>
 
+<script>
+  window.va = window.va || function () { (window.vaq = window.vaq || []).push(arguments); };
+</script>
+<script defer src="/_vercel/insights/script.js"></script>
 </body>
 </html>