feat(pwa): add screenshots to manifest for richer PWA install UI

Add wide (1024x515) and narrow (390x844) screenshots to the web app
manifest so Chrome displays the richer install UI with a preview on
both desktop and mobile. Register screenshots directory in
web_accessible_resources so Chrome can fetch the images from the
extension origin.

extension/dashboard/manifest.webmanifest

@@ -6,6 +6,22 @@
   "display": "standalone",
   "background_color": "#0f1117",
   "theme_color": "#0f1117",
+  "screenshots": [
+    {
+      "src": "screenshots/wide.png",
+      "sizes": "1024x515",
+      "type": "image/png",
+      "form_factor": "wide",
+      "label": "Holesail Browser Dashboard"
+    },
+    {
+      "src": "screenshots/narrow.png",
+      "sizes": "390x844",
+      "type": "image/png",
+      "form_factor": "narrow",
+      "label": "Holesail Browser Dashboard"
+    }
+  ],
   "icons": [
     { "src": "../icons/48.png",  "sizes": "48x48",   "type": "image/png" },
     { "src": "../icons/128.png", "sizes": "128x128", "type": "image/png" },