feat(tv): add corner spotlight backdrop option (#1628)

New TV-only Appearance toggle that pins the spotlight artwork to the
top-right corner (68% width, 72% height) with the left and bottom edges
feathered into the scaffold background, so the logo, metadata, and
shelves sit on a calm surface instead of the image. Default is off —
the full-bleed spotlight stays exactly as it is.

The artwork keeps its full-screen request size: the corner box only
crops the layout, so transcode URLs and image caches are unchanged and
toggling the setting stays instant.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/services/settings_service.dart

@@ -468,6 +468,7 @@ class SettingsService extends BaseSharedPreferencesService {
 
   static const bufferSize = _BufferSizePref();
   static const libraryDensity = _LibraryDensityPref();
+  static const tvCornerSpotlightBackdrop = BoolPref('tv_corner_spotlight_backdrop');
   static const episodePosterMode = _EpisodePosterModePref();
   static const continueWatchingAction = EnumPref<ContinueWatchingAction>(
     'continue_watching_action',
@@ -859,6 +860,7 @@ class SettingsService extends BaseSharedPreferencesService {
     themeMode,
     keyboardHotkeys,
     libraryDensity,
+    tvCornerSpotlightBackdrop,
     episodePosterMode,
     mediaVersionPreferences,
     localLastPlayedAt,