fix(plex): an episode with no number is named by its title on every card

The cards, the up-next card and the mark-next label printed a question
mark where Plex has no number; the player's header already showed the
title alone. One helper (episodeNumbers/episodeName) for every place.

frontend/src/lib/plexDetail.ts

@@ -205,20 +205,31 @@ export function useCrossfadeBackdrop(
 // Series detail pages (show + season subpage): the S·E label under a resume/next-up episode.
 export function epLabel(ep?: PlexCard | null): string | undefined {
   if (!ep) return undefined;
-  if (ep.season_number != null && ep.episode_number != null)
-    return `S${ep.season_number} · E${ep.episode_number}`;
-  return ep.title;
+  const n = episodeNumbers(ep);
+  return n ? `S${n.season} · E${n.episode}` : ep.title;
 }
 
-/** An episode's line under its show's name: "S1·E4 — Title" — or the title alone when Plex has no
- *  number for it (a special): a template literal printed the missing one as "Enull", JSX as "E". */
-export function episodeLine(ep: {
-  season_number?: number | null;
-  episode_number?: number | null;
-  title: string;
-}): string {
-  if (ep.season_number == null || ep.episode_number == null) return ep.title;
-  return `S${ep.season_number}·E${ep.episode_number} — ${ep.title}`;
+type Numbered = { season_number?: number | null; episode_number?: number | null };
+
+/** An episode's season and episode number, or null when Plex has no number for one of them (a
+ *  special). Every place that names an episode asks this, so a numberless one is named by its title
+ *  alone everywhere: a template literal printed the missing number as "Enull", JSX as "E", and a
+ *  `?? "?"` as "E?", which told the viewer nothing. */
+export function episodeNumbers(ep: Numbered): { season: number; episode: number } | null {
+  if (ep.season_number == null || ep.episode_number == null) return null;
+  return { season: ep.season_number, episode: ep.episode_number };
+}
+
+/** An episode named in a line of its own ("Next: …", "Mark … watched"): "S1·E4", or its title. */
+export function episodeName(ep: Numbered & { title: string }): string {
+  const n = episodeNumbers(ep);
+  return n ? `S${n.season}·E${n.episode}` : ep.title;
+}
+
+/** An episode's line under its show's name: "S1·E4 — Title", or the title alone (see above). */
+export function episodeLine(ep: Numbered & { title: string }): string {
+  const n = episodeNumbers(ep);
+  return n ? `S${n.season}·E${n.episode} — ${ep.title}` : ep.title;
 }
 
 // Plex names seasons in the library's language ("Season 1", "Specials"), so a Hungarian reader got