fix(plex): an episode Plex has no number for is named by its title alone

The OS media title printed the missing number as "Enull", the player and info headers as a bare
"E". One helper for the three lines.

frontend/src/lib/plexDetail.episodeLine.test.ts

@@ -0,0 +1,20 @@
+// @vitest-environment jsdom
+import { describe, expect, it } from "vitest";
+import { episodeLine } from "./plexDetail";
+
+describe("episodeLine", () => {
+  it("names the episode by its season and number", () => {
+    expect(episodeLine({ season_number: 2, episode_number: 7, title: "Pilot" })).toBe(
+      "S2·E7 — Pilot",
+    );
+  });
+
+  it("is the title alone for an episode Plex has no number for", () => {
+    expect(episodeLine({ season_number: 1, episode_number: null, title: "Special" })).toBe(
+      "Special",
+    );
+    expect(episodeLine({ season_number: null, episode_number: 3, title: "Special" })).toBe(
+      "Special",
+    );
+  });
+});

frontend/src/lib/plexDetail.ts

@@ -210,6 +210,17 @@ export function epLabel(ep?: PlexCard | null): string | undefined {
   return 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}`;
+}
+
 // Plex names seasons in the library's language ("Season 1", "Specials"), so a Hungarian reader got
 // English there. A generic name is rebuilt from the number in the reader's language; a season
 // with a name of its own ("Book One: Water") keeps it.