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/main.dart

@@ -179,10 +179,10 @@ Future<void> _bootstrapApp() async {
     }
   }
 
-  // Initialize TV detection (Android leanback or Apple TV) and PiP on Android.
-  if (Platform.isAndroid || Platform.isIOS) {
-    futures.add(TvDetectionService.getInstance(forceTv: settings.read(SettingsService.forceTvMode)));
-  }
+  // Initialize TV detection on every platform: auto-detect covers Android
+  // leanback and Apple TV; the force-TV setting applies anywhere, including
+  // desktop home-theater setups.
+  futures.add(TvDetectionService.getInstance(forceTv: settings.read(SettingsService.forceTvMode)));
   // Visual-effects tier (auto-detects low-end Android; full elsewhere).
   futures.add(DevicePerformance.getInstance(override: settings.read(SettingsService.visualEffects)));
   if (Platform.isAndroid) {