feat(electron): separate dev/uat build targets with isolated update channels

- Add build.dev.json to override publish.url for dev environment
- Add build:dev and build:uat scripts; each sets its own feed URL
- copy-to-nas.js now accepts env argument; outputs to
  electron-update/{dev|uat}/ locally and on NAS
- docker-compose volume mounts updated to serve from env subdirs
- Remove stale artifacts from electron-update root

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

docker-compose.dev.yml

@@ -7,7 +7,7 @@ services:
     ports:
       - "7033:80"
     volumes:
-      - ./electron-update:/usr/share/nginx/html/electron-update
+      - ./electron-update/dev:/usr/share/nginx/html/electron-update
     environment:
       - NODE_ENV=production
     restart: unless-stopped