feat(electron): add persistent auth, auto-updater, and NAS copy script

- Integrate electron-store for persistent PocketBase auth backup
  across localStorage clears and app reinstalls
- Switch build target from portable to nsis to generate latest.yml
  for electron-updater generic provider
- Add user confirmation dialogs before download and before install
- Add post-build script to copy .exe/.yml/.nupkg to NAS share and
  local electron-update/ directory for nginx volume mount
- Mount ./electron-update into frontend nginx containers via
  docker-compose for automatic update file serving

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

frontend/nginx.conf

@@ -39,6 +39,14 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
     }
 
+    # Electron 自动更新文件托管
+    # 部署时将 electron/dist/latest.yml 和 electron/dist/*.exe 放到此目录
+    location /electron-update/ {
+        alias /usr/share/nginx/html/electron-update/;
+        autoindex on;
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+
     # API 代理到局域网 PocketBase
     location /api/ {
         client_max_body_size 500m;