feat: add PWA support (installable app)

- Web app manifest with name, icons, theme color, standalone display
- Service worker with stale-while-revalidate caching strategy
- 192px and 512px PNG icons generated from favicon.svg
- Apple-specific meta tags for iOS home screen support
- Register service worker on page load

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

src/main.jsx

@@ -15,3 +15,10 @@ function Root() {
 }
 
 createRoot(document.getElementById('root')).render(<Root />);
+
+// Register service worker for PWA
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(() => {});
+  });
+}