Delete extension, OS backends, standalone server, root npm project

Removes extension/, desktop-client/backends/, desktop-client/config.js,
server/, root package.json/package-lock.json/node_modules/, and the
now-unused electron-store dependency from desktop-client/package.json.
The desktop-client is now a self-contained Electron app with no external
deps beyond express and ws.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

desktop-client/package.json

@@ -24,7 +24,6 @@
     }
   },
   "dependencies": {
-    "electron-store": "^8.1.0",
     "express": "^5.2.1",
     "ws": "^8.20.0"
   },