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>

.changes/portal-season-posters.md

@@ -0,0 +1,10 @@
+---
+type: feature
+area: portal
+highlight: Season posters
+---
+
+Series pages now show each season's own poster next to the season tabs, and
+the fullscreen episode list shows it above its tabs too. The picture comes
+from TMDB when metadata enrichment is on, otherwise from your provider's
+season art; one-season shows keep the page as it was.