feat(plex): expand shows to episodes so a show-collection's Play-all plays (S4)

A collection's Play-all/Shuffle queue held only playable leaves
(movies/episodes), filtering out shows — so on a show-collection the queue
was empty and Play-all was hidden/dead. Build the queue by expanding each
show member into its episodes (season then episode order), concatenated in
member order; leaves still queue as themselves. Shows are fetched lazily on
click (parallel plexShow fetches), not on page load. Gate Play-all/Shuffle
and the poster on "has any playable member" (leaves OR shows), and disable
them while the queue builds. Scope: option A — no member manual-order
migration. Adds plex.collection.playFailed (HU/EN).

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

@@ -23,6 +23,7 @@
     "added": "Added",
     "deleteConfirm": "Delete the collection \"{{title}}\"? This removes it from Plex too.",
     "editFailed": "Collection edit failed",
+    "playFailed": "Couldn't build the play queue — try again.",
     "newCollection": "New collection",
     "namePlaceholder": "Collection name",
     "nameTaken": "A collection with this name already exists.",

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

@@ -23,6 +23,7 @@
     "added": "Hozzáadva",
     "deleteConfirm": "Törlöd a(z) „{{title}}” kollekciót? Ez a Plexből is eltávolítja.",
     "editFailed": "A kollekció szerkesztése sikertelen",
+    "playFailed": "Nem sikerült összeállítani a lejátszási sort — próbáld újra.",
     "newCollection": "Új kollekció",
     "namePlaceholder": "Kollekció neve",
     "nameTaken": "Már létezik ilyen nevű kollekció.",