Initialize TvDetectionService on every platform so the existing
force_tv_mode setting drives the 10-foot TV interface on Windows,
macOS, and Linux, and surface the toggle in appearance settings there.
Desktop keeps real-mouse behavior in TV mode: InputModeTracker still
flips between pointer and keyboard modes (cursor hidden while
keyboard-driven), segmented controls keep their hover affordance, and
the settings backup section stays available (only Android TV lacks a
document picker).
Adds PlatformDetector.debugSetIsDesktopOSOverride so TV-device
simulations in widget tests don't inherit the desktop test host's
platform.
close#1409
edde746