feat(pwa): add service worker and scope for full PWA installability

Add a minimal sw.js with an empty fetch handler — required by Chrome
for the PWA install prompt to appear. Register it at the bottom of
dashboard.html with a silent catch. Add "scope": "." to the manifest.
Register sw.js in web_accessible_resources so Chrome can fetch it from
the extension origin.

extension/dashboard/manifest.webmanifest

@@ -4,6 +4,7 @@
   "short_name": "Holesail",
   "description": "P2P Holesail tunnels in the browser",
   "start_url": "dashboard.html",
+  "scope": ".",
   "display": "standalone",
   "display_override": ["window-controls-overlay", "standalone"],
   "background_color": "#0f1117",