fix(player): render subtitle tracks on the Linux HTML5 path (UR-020)

Selecting a subtitle on Linux did nothing. VideoPlayer rendered no <track>
children at all — the block was commented out as "temporarily disabled to
debug playback issues" (it has been that way since the POC) — so
Html5PlayerAdapter.selectSubtitle() walked an empty textTracks list and the
menu, which is built from media.mediaStreams, was purely decorative.

The reason it had to be disabled is still visible in the dead markup:
getSubtitleUrl() is async, so src={getSubtitleUrl(track.index)} bound a
Promise to the attribute and every track pointed at "[object Promise]" — an
unloadable resource hanging off the media element.

Subtitle URLs are now resolved off the render path into component state
(subtitleTracks.ts), and only streams whose URL actually resolved are
rendered; a per-track failure drops that track instead of emitting a dead
src. data-stream-index is kept, since that is what the adapter matches on.

Subtitles stay OFF unless the user asks for them: the server's isDefault flag
is shown in the menu but is never promoted to a selection, and the `default`
attribute is deliberately not emitted. A <track default> auto-shows, so the
menu would open on "Off" while subtitles were burned over the picture, and
every user who never wanted subtitles would suddenly get them. That matches
the existing initial state (selectedSubtitleIndex = null).

Selection and rendered tracks are reconciled whenever the list changes: a
selection that no longer resolves collapses to "Off", and a surviving one is
re-applied after the new <track> elements exist. "Off" disables every text
track, as before.

Cross-origin text-track fetches use the media element's CORS setting, so the
element opts in with crossorigin="anonymous" — but only for an http(s)
stream, never for a local/offline file:/asset: source, where forcing CORS
onto the video fetch could break playback. It is keyed on the subtitle stream
count, known at first render, so the attribute cannot flip under an in-flight
media load.

Android/native is untouched: the ExoPlayer branch still goes through
player_set_subtitle_track.

Tests (UT-143, UT-144) were written first and failed against the old markup:
the commented-out block, the Promise bound to src, and the default attribute.

TRACES: UR-020 | DR-023 | UT-143, UT-144

src/lib/components/player/subtitleTracks.ts

@@ -0,0 +1,151 @@
+// Subtitle plumbing for the Linux / WebKitGTK HTML5 `<video>` playback path.
+//
+// Extracted from VideoPlayer.svelte so it is unit-testable, and because the
+// original inline version hid a fatal mistake in plain sight: `getSubtitleUrl()`
+// is async, so `src={getSubtitleUrl(track.index)}` bound a *Promise* to the
+// attribute and every `<track>` pointed at "[object Promise]". The whole block
+// was commented out rather than fixed, which left `<video>` with no text tracks
+// at all — `Html5PlayerAdapter.selectSubtitle()` then iterated an empty
+// `textTracks` list and the subtitle menu silently did nothing.
+//
+// The rule this module enforces: URLs are resolved to plain strings *here*, off
+// the render path, and only tracks that actually resolved are handed to the
+// markup.
+//
+// TRACES: UR-020 | DR-023 | UT-143, UT-144
+
+/**
+ * The subset of `MediaStream` (from the generated bindings) this module needs.
+ * Kept structural so tests do not have to build full binding objects.
+ */
+export interface SubtitleStreamLike {
+  index: number;
+  kind?: string | null;
+  language?: string | null;
+  displayTitle?: string | null;
+  isDefault?: boolean;
+  isForced?: boolean;
+}
+
+/** A subtitle stream whose URL resolved — i.e. one we can actually render. */
+export interface RenderableSubtitleTrack {
+  /** Jellyfin media-stream index; the adapter matches `data-stream-index`. */
+  streamIndex: number;
+  /** Fully resolved WebVTT URL. Always a string, never a Promise. */
+  url: string;
+  srclang: string;
+  label: string;
+  /** Server's "default" flag — shown in the menu, never auto-enabled. */
+  isDefault: boolean;
+}
+
+/** Subtitle streams of a media item, in stream order. */
+export function subtitleStreamsOf(
+  streams: readonly SubtitleStreamLike[] | null | undefined,
+): SubtitleStreamLike[] {
+  if (!streams) return [];
+  return streams.filter((s) => s.kind === "subtitle");
+}
+
+/** Human label for a subtitle stream, matching the menu's own fallback chain. */
+export function subtitleTrackLabel(stream: SubtitleStreamLike): string {
+  return stream.displayTitle || stream.language || `Track ${stream.index}`;
+}
+
+/** A src we are willing to put on a `<track>`: a non-blank plain string. */
+function isRenderableUrl(url: unknown): url is string {
+  return typeof url === "string" && url.trim().length > 0;
+}
+
+/**
+ * Resolve every subtitle stream's URL and return only the tracks that can be
+ * rendered. `resolveUrl` failures are swallowed per track: one unavailable
+ * subtitle must not cost the user the others, and a dead `src` on a media
+ * element is exactly what made this block get disabled in the first place.
+ */
+export async function resolveSubtitleTracks(
+  streams: readonly SubtitleStreamLike[] | null | undefined,
+  resolveUrl: (streamIndex: number) => Promise<string>,
+): Promise<RenderableSubtitleTrack[]> {
+  const subtitles = subtitleStreamsOf(streams);
+  if (subtitles.length === 0) return [];
+
+  const resolved = await Promise.all(
+    subtitles.map(async (stream) => {
+      try {
+        const url = await resolveUrl(stream.index);
+        if (!isRenderableUrl(url)) return null;
+        return {
+          streamIndex: stream.index,
+          url,
+          srclang: stream.language || "und",
+          label: subtitleTrackLabel(stream),
+          isDefault: stream.isDefault === true,
+        } satisfies RenderableSubtitleTrack;
+      } catch {
+        return null;
+      }
+    }),
+  );
+
+  return resolved.filter((t): t is RenderableSubtitleTrack => t !== null);
+}
+
+/**
+ * The selection to keep once the rendered track list changes.
+ *
+ * Subtitles are OFF unless the user turns them on: `null` in, `null` out. The
+ * server's `isDefault` flag is deliberately NOT promoted to a selection (and the
+ * markup deliberately omits the `default` attribute, which would auto-show the
+ * track) — the menu opens on "Off", so auto-enabling would make the UI lie about
+ * what is on screen, and it would change behaviour for every user who has never
+ * asked for subtitles.
+ *
+ * A selection that is no longer renderable (new item, or a URL that failed to
+ * resolve) collapses to off, so the menu's checkmark can never point at a track
+ * that does not exist on the element.
+ */
+export function reconcileSelectedSubtitle(
+  tracks: readonly RenderableSubtitleTrack[],
+  selected: number | null,
+): number | null {
+  if (selected === null) return null;
+  return tracks.some((t) => t.streamIndex === selected) ? selected : null;
+}
+
+function originOf(url: string): string | null {
+  try {
+    const parsed = new URL(url);
+    if (parsed.protocol !== "http:" && parsed.protocol !== "https:") return null;
+    return parsed.origin;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * The `crossorigin` value for the `<video>` element, or undefined for none.
+ *
+ * Text-track fetches are CORS-enabled per the HTML spec and use the *media
+ * element's* CORS setting, so a cross-origin `<track>` never loads unless the
+ * element opts in. The webview page's origin is `tauri://localhost`, so every
+ * subtitle served by Jellyfin is cross-origin.
+ *
+ * Opting in is only safe when the media itself comes from an http(s) server —
+ * the same Jellyfin that already answers hls.js' cross-origin XHRs, so we know
+ * it sends the headers. For a local/offline source (`file:`/`asset:`) we leave
+ * the attribute off: subtitles staying dark there is the status quo, whereas
+ * forcing CORS onto the video fetch could break playback outright.
+ *
+ * Deliberately keyed on the *count of subtitle streams* rather than on the
+ * resolved tracks: both inputs are known at first render, so the attribute is
+ * decided before the element starts loading and never flips underneath an
+ * in-flight media fetch.
+ */
+export function videoCrossOriginMode(
+  streamUrl: string,
+  subtitleStreamCount: number,
+): "anonymous" | undefined {
+  if (subtitleStreamCount <= 0) return undefined;
+  return originOf(streamUrl) ? "anonymous" : undefined;
+}