fix(pwa): resolve proxy connection reset and manifest same-origin warnings

Fix "Proxy error: connection reset by peer" caused by dashboard-server.js
looking for files in native-host/ instead of extension/dashboard/. Remove
the devBasePath parameter and resolve DEV_DASHBOARD_DIR directly from
__dirname so dev and distribution mode both find the correct files.

Fix all manifest warnings (start_url/id/scope/protocol_handlers ignored)
by reverting manifest.webmanifest to relative/extension-origin URLs for
the chrome-extension:// context, and having dashboard-server.js dynamically
generate a manifest at /manifest.webmanifest with absolute
https://my.dash.board/ URLs — satisfying Chrome's same-origin requirement
for PWA installability.

Also serve extension icons at https://my.dash.board/icons/<file> with
correct bundle key mapping, and embed them in the distributable binary
via build-distributable.js.

extension/dashboard/manifest.webmanifest

@@ -1,10 +1,10 @@
 {
-  "id": "https://my.dash.board/",
+  "id": "/dashboard/dashboard.html",
   "name": "Holesail Browser",
   "short_name": "Holesail",
   "description": "P2P Holesail tunnels in the browser",
-  "start_url": "https://my.dash.board/",
-  "scope": "https://my.dash.board/",
+  "start_url": "dashboard.html",
+  "scope": ".",
   "display": "standalone",
   "display_override": ["window-controls-overlay", "standalone"],
   "background_color": "#0f1117",
@@ -12,7 +12,7 @@
   "protocol_handlers": [
     {
       "protocol": "web+holesail",
-      "url": "https://my.dash.board/?lookup=%s"
+      "url": "dashboard.html?lookup=%s"
     }
   ],
   "screenshots": [