Use Electron system directory picker

[wisdomqin]

What changed

• route workspace directory selection through Electron dialog.showOpenDialog
• expose a narrow preload bridge for the Harness UI
• remove the Windows Koffi/native worker path and obsolete browse fallback patch
• restrict picker IPC requests to the main Harness frame

Why

Packaged Windows builds could report win32 folder dialog worker exited before reporting a result. The Electron main process already owns the native window, so it should own the system directory dialog as well.

User impact

Windows and macOS now use their normal system folder picker without the crash-prone worker process.

Validation

• npm run typecheck
• npm test (71 tests)
• npm run build
• npm run package:dev:dir
• clean npm ci --ignore-scripts followed by npm run postinstall