refactor(plex): share showEpisodeKeys + honest empty Play-all

Extract the show->ordered-episode-keys flatten into one pure, unit-tested
helper (lib/plexQueue.ts) and use it at all three sites — the collection
Play-all (S4) and the show/season play-through pages in plexViews — so the
season-then-episode queue-order invariant lives in one place instead of three
hand-rolled copies. Behaviour-identical (the helper's ?? [] is a safe superset
of the collection site's previously non-null read).

Also: a show-collection whose shows expand to zero episodes had an enabled
Play-all that silently no-op'd — the same 'does nothing' state S4 removed for
the empty-queue case. Surface an honest info notice (plex.collection.playEmpty,
HU+EN) instead.

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

@@ -24,6 +24,7 @@
     "deleteConfirm": "Delete the collection \"{{title}}\"? This removes it from Plex too.",
     "editFailed": "Collection edit failed",
     "playFailed": "Couldn't build the play queue — try again.",
+    "playEmpty": "Nothing to play in this collection yet.",
     "newCollection": "New collection",
     "namePlaceholder": "Collection name",
     "nameTaken": "A collection with this name already exists.",

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

@@ -24,6 +24,7 @@
     "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.",
+    "playEmpty": "Ebben a kollekcióban még nincs mit lejátszani.",
     "newCollection": "Új kollekció",
     "namePlaceholder": "Kollekció neve",
     "nameTaken": "Már létezik ilyen nevű kollekció.",