Add global shortcuts with press and release

Apps can register shortcuts that fire while any app has focus and see both the press and the release, so hold-to-talk works. The Windows host is written but was not compiled or run locally. Closes #45.

Before

Shortcuts only fire while the app's own window has focus, and there is no release event.

https://github.com/user-attachments/assets/1db19f52-9872-448e-a1ab-1f50fbc60223

After

Sub.globalShortcut registers a system-wide shortcut on macOS and Windows; a fixture core and its e2e test cover press, release, and rejection.

https://github.com/user-attachments/assets/e6587937-835a-44bb-9cbb-96a18658040c