feat: add service worker for PWA support (offline caching, installability)

- Add sw.js with stale-while-revalidate caching for static assets
- Network-first for HTML navigation, skip API/WS requests
- Register service worker in main.tsx on page load
- Enhanced manifest.json with orientation, categories, and all icon sizes
- App is now installable as a standalone PWA on mobile and desktop

src/main.tsx

@@ -5,6 +5,15 @@ import { ErrorBoundary } from './components/ErrorBoundary'
 import { ThemeProvider } from './contexts/ThemeContext'
 import './index.css'
 
+// Register service worker for PWA support (offline caching, installability)
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(() => {
+      // SW registration failed — app works fine without it
+    });
+  });
+}
+
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <ErrorBoundary>