feat(plex): Plex-web-style 3-level series view (Phase 2 of series view)

Restructure TV browsing into show detail → seasons → season episodes → player, like
the Plex web app.

Backend: /show/{rk} now returns a rich show page — hero meta (rating/content_rating/
genres/studio + live IMDb), live Cast & Crew, Related shows (Plex 'related', mapped to
our mirrored shows so they're openable), and per-season cards with an aggregate
watch-state + on-deck episode, plus show-level resume (on-deck)/first(play-from-start)/
status rollup. New PlexClient.related(). New bulk-state endpoints POST /show/{rk}/state
and /season/{rk}/state mark every episode watched/unwatched for the user and mirror
each change to a linked Plex account in the background (best-effort, checked once).

Frontend: PlexShowView reworked into the show detail page (hero + Resume/Play-from-start/
Mark-show-watched/Add-to-playlist/[admin]Add-to-collection + season card grid + cast +
related strips); new PlexSeasonView season subpage (hero + Resume/Play/Mark-season/
Add-season-to-playlist + landscape episode grid). Both read the one cached ['plex-show']
payload (season page picks its season out of it — instant, no extra fetch). Player queue =
the whole show (from the show page) or the season (from the season page) so prev/next +
auto-advance follow order. New 'season' history subview; Backspace steps back one drill
level (grid←show←season, out of info/playlist) alongside browser/mouse Back. Season-level
'Add to collection' intentionally omitted (Plex collections hold whole shows, not seasons).
i18n plex.series.* (en/hu/de).

backend/app/plex/client.py

@@ -96,6 +96,18 @@ class PlexClient:
         mc = self._get(f"/library/metadata/{rating_key}/children")
         return mc.get("Metadata", []) or []
 
+    def related(self, rating_key: str) -> list[dict]:
+        """Related / similar items for a movie or show, flattened out of Plex's Hub grouping.
+        Best-effort (needs the library's related data) — returns [] if Plex has none."""
+        try:
+            mc = self._get(f"/library/metadata/{rating_key}/related")
+        except PlexError:
+            return []
+        out: list[dict] = []
+        for hub in mc.get("Hub", []) or []:
+            out.extend(hub.get("Metadata", []) or [])
+        return out
+
     def collections(self, section_key: str) -> list[dict]:
         """All collections in a library section (title/summary/thumb/childCount/smart)."""
         mc = self._get(f"/library/sections/{section_key}/collections")