feat(portal): show each season's own poster beside the season tabs (#1628)

Series detail pages now render the selected season's poster as a season
cover next to the season tabs and description, and the fullscreen episode
panel shows the same poster as a season strip above its tabs.

Resolution is TMDB-first, like the show artwork merge: the lazy season
enrichment stores `/tv/{id}/season/{n}` `poster_path` as a w342 URL in
`tmdb_season_posters` (Xtream) or `StalkerSeriesTmdbSeasonsService.posters()`
(Stalker), under the same write-only-if-changed convergence guard as the
season overview. Xtream falls back to the provider's `seasons[].cover_big`/
`cover` when it is an http(s) URL other than the show poster, because panels
repeat the show poster on every season. Stalker is TMDB-only.

The cover column is not rendered for one-season items, seasons without a
poster, or a failed image, so every fallback is today's markup. It is sized
by a new `--season-cover-width` token (96/120/144px per Settings.coverSize).
The hero poster never follows the season.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/xtream-serie-details.interface.ts

@@ -17,6 +17,13 @@ export interface XtreamSerieDetails {
      * the provider's `seasons[].overview` is empty or URL-only junk.
      */
     tmdb_season_overviews?: Record<string, string>;
+    /**
+     * Populated by lazy TMDB season enrichment; absent in raw provider
+     * responses. Keyed like `tmdb_season_overviews`. Full image URLs of
+     * each season's own TMDB poster, the first choice for the season
+     * cover in the detail view and the fullscreen episode panel.
+     */
+    tmdb_season_posters?: Record<string, string>;
 }
 
 export interface XtreamSerieInfo {