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.

app/(auth)/(tabs)/(home)/settings.tv.tsx

@@ -854,6 +854,13 @@ export default function SettingsTV() {
               updateSettings({ mergeNextUpAndContinueWatching: value })
             }
           />
+          <TVSettingsToggle
+            label={t("home.settings.appearance.use_episode_images_next_up")}
+            value={settings.useEpisodeImagesForNextUp}
+            onToggle={(value) =>
+              updateSettings({ useEpisodeImagesForNextUp: value })
+            }
+          />
           <TVSettingsToggle
             label={t("home.settings.appearance.show_home_backdrop")}
             value={settings.showHomeBackdrop}