feat: extend Force TV mode to desktop for home-theater setups

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

lib/focus/input_mode_tracker.dart

@@ -117,8 +117,10 @@ class _InputModeTrackerState extends State<InputModeTracker> {
   Widget build(BuildContext context) {
     // On Android TV, don't switch to pointer mode from pointer events
     // as D-pad can generate synthetic pointer events that would incorrectly
-    // trigger pointer mode and show a cursor instead of D-pad focus navigation
-    if (TvDetectionService.isTVSync()) {
+    // trigger pointer mode and show a cursor instead of D-pad focus navigation.
+    // Desktop is exempt even in force-TV mode: its pointer events come from a
+    // real mouse, which should keep flipping modes (and the cursor) as usual.
+    if (TvDetectionService.isTVSync() && !PlatformDetector.isDesktopOS()) {
       return _InputModeProvider(mode: _mode, child: widget.child);
     }