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/components/PlexCollectionView.tsx

@@ -6,6 +6,8 @@ import { qk } from "../lib/queryKeys";
 import { api, isIntrinsicReadOnly, type PlexCard, type PlexCollectionDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
 import { notifyActionError, notifyCollectionEditError } from "../lib/actionErrors";
+import { notify } from "../lib/notifications";
+import { showEpisodeKeys } from "../lib/plexQueue";
 import { useConfirm } from "./ConfirmProvider";
 import { BackBtn, ActionBtn, DetailHero } from "../lib/plexDetailUi";
 import { PlexPosterCard } from "./plexCards";
@@ -87,7 +89,7 @@ export default function PlexCollectionView({
           queryKey: qk.plexShow(c.id),
           queryFn: () => api.plexShow(c.id),
         });
-        return detail.seasons.flatMap((s) => s.episodes.map((e) => e.id));
+        return showEpisodeKeys(detail);
       }),
     );
     return perMember.flat();
@@ -98,7 +100,10 @@ export default function PlexCollectionView({
     try {
       let queue = await buildQueue();
       if (shuffle) queue = shuffled(queue);
+      // canPlay gates on "has a show member", but a show can expand to zero episodes (unmatched /
+      // empty). An enabled Play-all that silently no-ops is a dishonest state, so say so plainly.
       if (queue.length > 0) onPlay(queue[0]!, queue);
+      else notify({ level: "info", message: t("plex.collection.playEmpty") });
     } catch (e) {
       notifyActionError(e, t("plex.collection.playFailed"));
     } finally {

frontend/src/components/plexViews.tsx

@@ -8,6 +8,7 @@ import { plexLang } from "../i18n";
 import { pickRatingBadge } from "../lib/plexRating";
 import { notifyActionError } from "../lib/actionErrors";
 import { toggleWatchState } from "../lib/plexWatch";
+import { showEpisodeKeys } from "../lib/plexQueue";
 import {
   ActionBtn,
   BackBtn,
@@ -306,7 +307,7 @@ export function PlexShowView({
 
   const show = d?.show;
   const showLib = show?.library ?? undefined;
-  const allKeys = (d?.seasons ?? []).flatMap((se) => se.episodes.map((e) => e.id));
+  const allKeys = showEpisodeKeys(d);
   const watched = show?.status === "watched";
   const {
     artBg,
@@ -568,7 +569,7 @@ export function PlexSeasonView({
   // Play-through queue spans the WHOLE show (all seasons, in order) so a season's last episode rolls
   // straight into the next season's first — the same cross-season order the show page already uses.
   // seasonKeys stays only for the season-scoped "add to playlist" action.
-  const allKeys = (d?.seasons ?? []).flatMap((s) => s.episodes.map((e) => e.id));
+  const allKeys = showEpisodeKeys(d);
   const watched = se?.status === "watched";
   const { artBg, showCast, crossfade, toggleArtBg, toggleCast, toggleCrossfade } = useDetailPrefs();
   useCrossfadeBackdrop(

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.",

frontend/src/lib/plexQueue.test.ts

@@ -0,0 +1,32 @@
+import { describe, it, expect } from "vitest";
+import { showEpisodeKeys } from "./plexQueue";
+import type { PlexCard, PlexSeasonDetail, PlexShowDetail } from "./api";
+
+const ep = (id: string): PlexCard => ({ id }) as PlexCard;
+const season = (episodes: PlexCard[]): PlexSeasonDetail =>
+  ({ episodes }) as unknown as PlexSeasonDetail;
+const show = (seasons: PlexSeasonDetail[]): PlexShowDetail =>
+  ({ seasons }) as unknown as PlexShowDetail;
+
+describe("showEpisodeKeys", () => {
+  it("flattens seasons in order, episodes in order, concatenated across seasons", () => {
+    const detail = show([
+      season([ep("s1e1"), ep("s1e2")]),
+      season([ep("s2e1"), ep("s2e2"), ep("s2e3")]),
+    ]);
+    expect(showEpisodeKeys(detail)).toEqual(["s1e1", "s1e2", "s2e1", "s2e2", "s2e3"]);
+  });
+
+  it("returns [] for a show with no seasons", () => {
+    expect(showEpisodeKeys(show([]))).toEqual([]);
+  });
+
+  it("returns [] for a season with no episodes (the enabled-but-empty Play-all case)", () => {
+    expect(showEpisodeKeys(show([season([])]))).toEqual([]);
+  });
+
+  it("returns [] for null/undefined so an in-flight query's data passes straight through", () => {
+    expect(showEpisodeKeys(null)).toEqual([]);
+    expect(showEpisodeKeys(undefined)).toEqual([]);
+  });
+});

frontend/src/lib/plexQueue.ts

@@ -0,0 +1,10 @@
+import type { PlexShowDetail } from "./api";
+
+// The ordered episode rating-keys of a show: season order, then episode order within each season —
+// the play-queue order the show/season pages and the collection Play-all all rely on. Kept in ONE
+// place so that invariant (and any future explicit sort, e.g. pushing season 0 "Specials" last)
+// lives at a single site rather than the three hand-rolled `seasons.flatMap(...)` copies it grew
+// into. `detail` is nullable so callers can pass an in-flight query's `data` straight through.
+export function showEpisodeKeys(detail: PlexShowDetail | null | undefined): string[] {
+  return (detail?.seasons ?? []).flatMap((s) => s.episodes.map((e) => e.id));
+}