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/public/manifest.webmanifest

@@ -0,0 +1,38 @@
+{
+  "name": "maestro",
+  "short_name": "maestro",
+  "description": "Unified terminal-session orchestrator — see and attach to every shell and agent across your machines.",
+  "id": "/",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#1a1d24",
+  "theme_color": "#1a1d24",
+  "icons": [
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    }
+  ]
+}