Add `toggle_background_opacity` action for macOS

[liby]

Changes

• Added ghosttyBackgroundOpacityDidToggle notification for handling opacity toggle events

• Implemented opacity toggle logic in SurfaceView_AppKit.swift

• If background-opacity is 1 or greater, do nothing

Testing

1. Set background-opacity < 1 in config

2. Bind a key to toggle_background_opacity action

3. Press the key to toggle between transparent and opaque states

4. Verify that:

   • Window properly toggles between transparent and opaque states

   • Background color is correct in both states

   • Action does nothing when background-opacity is 1 or more

   • Action does nothing in fullscreen mode

https://github.com/user-attachments/assets/04f114e0-4035-4b8c-860f-71ef696f2258

Resolves #5047