refactor(desktop): 替换 WebUI 为 Electron + electron-vite 桌面壳方案

- 使用 electron-vite 构建 main/preload，electron-builder 打包分发
- main process: dev 模式直连 localhost:3000，生产模式 spawn sidecar binary
- 添加 loading 页面，server 就绪前显示加载动画
- 更新 catalog 依赖: electron, electron-vite, electron-builder
- 移除 @webui-dev/bun-webui 依赖

package.json

@@ -44,7 +44,9 @@
     "drizzle-kit": "^0.31.8",
     "drizzle-orm": "^0.45.1",
     "drizzle-zod": "^0.8.3",
-    "@webui-dev/bun-webui": "^2.5.4",
+    "electron": "^34.0.0",
+    "electron-builder": "^26.0.0",
+    "electron-vite": "^5.0.0",
     "nitro": "npm:nitro-nightly@3.0.1-20260206-171553-bc737c0c",
     "ohash": "^2.0.11",
     "postgres": "^3.4.8",