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/manifest.json

@@ -54,6 +54,7 @@
         "wrong-domain.html",
         "dashboard/dashboard.html",
         "dashboard/manifest.webmanifest",
+        "dashboard/sw.js",
         "dashboard/screenshots/*.png",
         "dashboard/dashboard.css",
         "dashboard/data/*.js",