fix(pwa): remove no-op fetch handler from service worker

Chrome warns that an empty fetch listener adds navigation overhead.
Removed the handler entirely — a registered SW without a fetch handler
still satisfies Chrome's PWA installability requirement since v112.

extension/dashboard/sw.js

@@ -1,3 +1,3 @@
 // Minimal service worker required for PWA installability.
-// No caching is needed — the extension bundle is already local.
-self.addEventListener('fetch', () => {});
+// No fetch handler needed — the extension bundle is already local
+// and Chrome does not require one for the install prompt since v112.