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/dashboard/manifest.webmanifest

@@ -0,0 +1,13 @@
+{
+  "name": "Holesail Browser",
+  "short_name": "Holesail",
+  "description": "P2P Holesail tunnels in the browser",
+  "start_url": "dashboard.html",
+  "display": "standalone",
+  "background_color": "#0f1117",
+  "theme_color": "#0f1117",
+  "icons": [
+    { "src": "../icons/48.png",  "sizes": "48x48",   "type": "image/png" },
+    { "src": "../icons/128.png", "sizes": "128x128", "type": "image/png" }
+  ]
+}