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/sw.js

@@ -0,0 +1,3 @@
+// Minimal service worker required for PWA installability.
+// No caching is needed — the extension bundle is already local.
+self.addEventListener('fetch', () => {});