fix(desktop): preload must be CJS when sandbox:true (Electron limitation)

Root cause: 'Renderer is not connected to the main process' was happening
because the preload script never executed. Electron's sandbox mode requires
preload scripts in CommonJS format — ESM (.mjs) preload + sandbox:true is
explicitly unsupported (see https://www.electronjs.org/docs/latest/tutorial/esm).

Fix:
- electron.vite.config: pin preload output to format:'cjs', entryFileNames:'index.js'
- main/index.ts: load 'index.js' (revert the earlier .mjs change)

Verified: window.codesign is now defined, custom base URL field can validate
against proxy endpoints (e.g. https://www.duckcoding.ai).

Signed-off-by: hqhq1025 <1506751656@qq.com>

Author: hqhq1025

apps/desktop/electron.vite.config.ts

@@ -17,6 +17,7 @@ export default defineConfig({
       outDir: 'out/preload',
       rollupOptions: {
         input: { index: resolve(__dirname, 'src/preload/index.ts') },
+        output: { format: 'cjs', entryFileNames: 'index.js' },
       },
     },
   },