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/manifest.json

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