Decouple immersive mode from keyboard capture

[themartiano]

Summary

• keep Cocoa full-grab enabled whenever the Omarchy window is focused, in immersive and standard presentation modes
• add an independent Cocoa immersive option that controls only fullscreen menu bar and Dock behavior
• start the windowed guest centered at 75% of the usable display in AppKit points while preserving 16:9
• update start-menu guidance, runtime patch checksums, documentation, and native contracts