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/dashboard.html

@@ -1293,5 +1293,12 @@
 
 <!-- Entry point (runs init()) -->
 <script src="core/init.js"></script>
+
+<!-- PWA service worker registration (required for installability) -->
+<script>
+if ('serviceWorker' in navigator) {
+  navigator.serviceWorker.register('sw.js').catch(() => {});
+}
+</script>
 </body>
 </html>

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",

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', () => {});

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",