feat(viewer): add a header-free desktop focus mode

[josdyr]

On small laptop screens the fixed header and margins reduce the space available for the simulator. Add Window → Focus Mode (Shift-Command-F): an ordinary borderless desktop window that fits the visible screen, retaining a five-point resize margin and the device aspect ratio. Toggle it off to restore the previous frame, native toolbar and window buttons. Command-Option-drag moves the window; existing menus and device shortcuts remain available.

Focus Mode is a global preference remembered across launches. Toggling updates all open simulator windows, newly opened devices follow it, and each device preserves its own normal frame. Existing per-device preferences migrate automatically; bezel visibility remains independent. It leaves simulator lifecycle and rendering/input transforms unchanged.

Validation:

• scripts/build-app.sh debug succeeded on Xcode 27.
• swift test --package-path engine passed, including focus layout and menu-checkmark coverage; simulator-gated integration cases were not enabled.
• Manually verified with an iOS 27 simulator: header removal, larger portrait viewport, tapping between app tabs, landscape/portrait rotation, and restoration of Home/Screenshot/Rotate toolbar buttons. Also verified full quit/relaunch restores Focus Mode, toggling off restores the toolbar, and the normal frame preference is unchanged. Native full-screen transitions and multi-display behavior have not been manually tested.