feat(appearance): episode images for Next Up & Continue Watching

- Fix black episode thumbnails in the Next Up / Continue Watching rows:
  build the parent Thumb URL from the matched ParentThumbItemId +
  ParentThumbImageTag pair, instead of pairing ParentBackdropItemId with
  the thumb tag (different parent -> 404 -> black). Fixed on mobile
  (ContinueWatchingPoster) and TV (TVPosterCard, TVHeroCarousel).
- Add a "Use episode images for Next Up & Continue Watching" setting
  (default off = series image, matching Jellyfin), wired into the home
  rows on mobile and TV.
- Add helper descriptions under the Appearance settings rows.

utils/atoms/settings.ts

@@ -273,6 +273,9 @@ export type Settings = {
   hideBrightnessSlider: boolean;
   usePopularPlugin: boolean;
   mergeNextUpAndContinueWatching: boolean;
+  // Use the episode's own image (instead of the series thumb) for the
+  // "Next Up" and "Continue Watching" home rows.
+  useEpisodeImagesForNextUp: boolean;
   // TV-specific settings
   showHomeBackdrop: boolean;
   showTVHeroCarousel: boolean;
@@ -376,6 +379,7 @@ export const defaultValues: Settings = {
   hideBrightnessSlider: false,
   usePopularPlugin: true,
   mergeNextUpAndContinueWatching: false,
+  useEpisodeImagesForNextUp: false,
   // TV-specific settings
   showHomeBackdrop: true,
   showTVHeroCarousel: true,