fix(plex): subtitles survive hls.js and a remounted track

hls.js emptied the cues of every text track on the element when a manifest
loaded or the media attached, our own <track> included, so a VTT that loaded
first showed nothing for the session; its subtitle and caption controllers are
no longer built. A remounted default <track> was switched to showing by the
browser after the hide had run, drawing the cues natively under ours; the hide
now follows every added track and mode change.

frontend/src/lib/hlsPort.test.ts

@@ -0,0 +1,42 @@
+// hls.js empties the cues of every text track on the <video> when a manifest loads or the media
+// attaches — the Plex player's own subtitle <track> included, so a VTT that loaded first showed
+// nothing for the rest of the session. Its subtitle and caption controllers are not built at all.
+import { describe, expect, it, vi } from "vitest";
+
+const built: Record<string, unknown>[] = [];
+vi.mock("hls.js", () => {
+  class FakeHls {
+    static isSupported = () => true;
+    static Events = {};
+    static ErrorTypes = {};
+    constructor(config: Record<string, unknown>) {
+      built.push(config);
+    }
+    on() {}
+  }
+  return { default: FakeHls };
+});
+
+const { createHlsPort } = await import("./hlsPort");
+
+describe("createHlsPort", () => {
+  it("builds hls.js without the controllers that clear every text track's cues", () => {
+    const noop = () => {};
+    createHlsPort({
+      onManifestParsed: noop,
+      onAudioTracksUpdated: noop,
+      onFragLoaded: noop,
+      onError: noop,
+    });
+    const config = built[0] ?? {};
+    for (const key of [
+      "subtitleTrackController",
+      "subtitleStreamController",
+      "timelineController",
+    ]) {
+      // Set, as undefined: hls.js spreads the config over its defaults, so an absent key keeps them.
+      expect(key in config).toBe(true);
+      expect(config[key]).toBeUndefined();
+    }
+  });
+});