feat(plex): every show card plays the episode it would put on

A show card's picture opened the show page everywhere but Home, whose shelves carried the
resolved `next`. Now every Plex picture plays: a show asks `GET /api/plex/show/{id}/next`
(the in-progress episode, else the first unwatched, else S1·E1 — the rule Home and the show
page's Resume follow) on the click, fresh each time. A started show shows Continue + Restart;
its title, an ⓘ and the menu's "Open show" row open its page; one with no episodes opens there.

The question is asked per show, never carried on every card: three probes a show would cost a
grid page up to 180 queries. The hover preview asks it once it opens and names the episode
("Next: S6·E16 — Title"). Related shows on a show page play the same way.

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

@@ -127,6 +127,7 @@
   "episodeLine": "S{{season}}·E{{episode}} · {{title}}",
   "seasonTitle": "Season {{n}}",
   "specials": "Specials",
+  "playShowFailed": "Couldn't find the episode to play — please try again",
   "watchedFailed": "Couldn't update the watched state — please try again",
   "seasons_one": "{{count}} season",
   "seasons_other": "{{count}} seasons",

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

@@ -127,6 +127,7 @@
   "episodeLine": "S{{season}}·E{{episode}} · {{title}}",
   "seasonTitle": "{{n}}. évad",
   "specials": "Különkiadások",
+  "playShowFailed": "Nem sikerült megtalálni a lejátszandó epizódot — próbáld újra",
   "watchedFailed": "Nem sikerült frissíteni a megnézett állapotot — próbáld újra",
   "seasons_one": "{{count}} évad",
   "seasons_other": "{{count}} évad",