feat(desktop): add new-thread keyboard shortcut

[serejja]

Summary

Adds a native new-thread shortcut to the desktop app.

• adds File -> New Thread with the CmdOrCtrl+N accelerator
• routes the menu action through a new main-to-renderer window channel instead of adding a second thread-creation path
• reuses the existing renderer createThread() launchpad flow so menu, keyboard, and sidebar entry points all land in the same behavior
• adds focused main-process and renderer coverage for the new channel, menu wiring, and launchpad handoff

Testing

• pnpm test apps/desktop/src/main/__tests__/window-open-new-thread.test.ts apps/desktop/src/main/__tests__/menu.test.ts apps/desktop/src/main/__tests__/index.test.ts apps/desktop/src/renderer/src/__tests__/app-shell.test.tsx