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/components/UpNextCard.dom.test.tsx

@@ -81,7 +81,9 @@ describe("UpNextCard", () => {
     expect(onAdvance).not.toHaveBeenCalled();
   });
 
-  it("an episode heading falls back to S?·E? when season/episode are null", async () => {
+  // An episode Plex has no number for (a special) is named by its title alone everywhere — a "?"
+  // told the viewer nothing, and the player's own header already shows the title alone.
+  it("an episode heading drops the numbers when Plex has none for it", async () => {
     (api.plexItem as ReturnType<typeof vi.fn>).mockResolvedValue({
       id: "e7",
       kind: "episode",
@@ -91,6 +93,8 @@ describe("UpNextCard", () => {
       episode_number: null,
     });
     renderCard();
-    expect(await screen.findByText("Westworld · S?·E?")).toBeTruthy();
+    expect(await screen.findByText("Westworld")).toBeTruthy();
+    expect(screen.getByText("The Adversary")).toBeTruthy();
+    expect(screen.queryByText(/\?/)).toBeNull();
   });
 });