Main Window Desktop Layer Tasks

Add EnableMainWindowDesktopLayer to app settings with a disabled default.
Add developer settings UI and localization strings.
Add confirmation flow for mutual exclusion with fused desktop.
Add a dedicated main-window desktop-layer service.
Wire main-window creation, restore, tray fallback, settings changes, and shutdown cleanup to the service.
Keep Air APP windows outside this layer service.
Add static regression tests for settings, restore paths, and service boundaries.
Perform manual Windows z-order validation with real apps.