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/screens/settings/settings_screen.dart

@@ -164,7 +164,9 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab, Moun
 
                 if (UpdateService.isUpdateCheckEnabled) ...[_buildUpdateSection()],
 
-                if (!PlatformDetector.isTV()) _buildBackupSection(),
+                // Hidden on Android TV / tvOS (no document picker); desktop in
+                // force-TV mode keeps it — FilePickerService works there.
+                if (!PlatformDetector.isTV() || PlatformDetector.isDesktopOS()) _buildBackupSection(),
 
                 const SizedBox(height: 24),
                 SettingsGroup(