feat(maestro): make web UI an installable PWA

Add a web app manifest, icons, and a runtime-caching service worker so the
UI can be installed on Android Chrome (and added to the iOS home screen).

- web/public/manifest.webmanifest: standalone display, theme/bg colors, icon set
- web/public/icon.svg + generated icon-192/512 and apple-touch-icon PNGs
- web/public/sw.js: caches the static shell for offline launch; never touches
  /api (HTTP or WS), so live-daemon behavior is unchanged
- index.html: link manifest + apple-touch-icon, iOS standalone metas
- main.ts: register the service worker (production only)

Assets land in web/dist via Vite's public/ dir and are embedded by the
existing include_dir! in maestro-web, so no Rust changes are needed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/main.ts

@@ -9,6 +9,19 @@ import "./styles.css";
 
 initTheme();
 installViewportSizing();
+registerServiceWorker();
+
+// Register the PWA service worker so the app is installable and the shell loads
+// offline. Production only — in dev the SW would fight Vite's HMR.
+function registerServiceWorker(): void {
+  if (import.meta.env.DEV) return;
+  if (!("serviceWorker" in navigator)) return;
+  window.addEventListener("load", () => {
+    void navigator.serviceWorker.register("/sw.js").catch(() => {
+      // Best-effort: a failed registration must never break the app.
+    });
+  });
+}
 
 const rootEl = document.getElementById("app");
 if (!rootEl) {