feat(pwa): add PWA install support and update installation docs

Add manifest.webmanifest to the dashboard so Chrome can install it as
a standalone app via the browser menu. Register the manifest in
web_accessible_resources and link it from dashboard.html. Add a
dedicated PWA install section to docs/INSTALLATION.md covering install
steps, uninstall, and the requirement that the extension remains
installed.

extension/manifest.json

@@ -53,6 +53,7 @@
         "vendor/novnc.js",
         "wrong-domain.html",
         "dashboard/dashboard.html",
+        "dashboard/manifest.webmanifest",
         "dashboard/dashboard.css",
         "dashboard/data/*.js",
         "dashboard/core/*.js",