Add PWA support for install-to-home-screen

Adds manifest.json, service worker with static asset caching,
resized cat icons, and meta tags for iOS/Android installability.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

raggr-frontend/src/index.tsx

@@ -11,3 +11,9 @@ if (rootEl) {
     </React.StrictMode>,
   );
 }
+
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(console.warn);
+  });
+}