PWA installability + working service worker (wishlist #8)

manifest.webmanifest with the existing icons, apple-touch-icon, and SW
registration at startup. The old sw.js was a self-unregistering cleanup
stub — meaning serviceWorker.ready never resolved and web push has been
silently dead; the new worker handles push display and notification
clicks, and makes the app installable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -3,6 +3,13 @@ import { createRoot } from "react-dom/client";
 import App from "./App";
 import "./index.css";
 
+if ("serviceWorker" in navigator) {
+  // Required for both web push and PWA installability.
+  navigator.serviceWorker.register("/sw.js").catch((err) => {
+    console.warn("Service worker registration failed:", err);
+  });
+}
+
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
     <App />