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",
+    );
+  });
+});