Series drill-down: season posters

The drill-down reads the series with includeSeasonImages=true (Sonarr
only sends season posters when asked, as TheTVDB links). A selected
season or episode shows its season's poster; without one, or when it
cannot be loaded, the series poster.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -12,6 +12,7 @@ A terminal UI to search for movies and series with [Radarr](https://radarr.video
   lists from [Trakt](https://trakt.tv): trending now, most watched this week, anticipated, popular,
   and the US box office for movies. `[` and `]` switch lists; titles already in your library are
   ticked, the others can be added right there (Enter). Needs a free Trakt client ID (see Configure).
+- **Season posters**: in a series' seasons and episodes, the poster follows the selected season.
 - **Cast**: with a Trakt client ID, the details pane also lists the director (or the creators of a
   series) and the cast with their roles, below the plot, in every view.
 - **Interactive search**: list the releases Radarr/Sonarr find (approved or rejected, with reasons),