Merge feature/ios-skin: iPhone springboard skin + OS auto-detection

Three skins routed by device: iPhone -> iOS springboard, Mac/iPad -> macOS,
Windows/Android/Linux -> Linux. ?os= override and the explicit switch choice
always beat detection. iOS: status bar, hero widget, icon grid, frosted dock,
full-screen app sheets over existing content, full-screen terminal.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: captkernel