fix(plex): series-page UX fixes from UAT (7 items)

1. Series detail pages now match the movie info page: the customize menu lives in the
   hero panel (top-right), the Cast/Seasons/Related strips sit in glassy panels, and the
   menu is dynamic — Related shows can be toggled (new plexInfoRelated pref).
2. The Plex search box now survives F5 (persisted per-account like the filters).
3. Show + season hero posters get a hover Play/Resume overlay.
4. Card top-right watched toggle can now UN-watch: the watched/in-progress badges were
   intercepting the click (no pointer-events-none), so the toggle underneath never fired.
5. Episode card: the watched check no longer jumps on hover (badge now shares the
   toggle's box); every card's title/meta gets its own glassy translucent background and
   the Add-to-playlist button is always visible (not hover-only).
6. Season-card 'In progress' label is readable again (solid dark badge over the poster).
7. On a season page the show title is clickable → back to the show (history-correct).

frontend/src/i18n/locales/en/plex.json

@@ -181,6 +181,7 @@
     "customize": "Customize view",
     "prefArtBg": "Faint backdrop",
     "prefCast": "Cast & crew",
+    "prefRelated": "Related shows",
     "stripSource": {
       "collection": "Collections",
       "imdb": "IMDb",