feat(playback): report Embedded MPV file chapters on the timeline (re-land #1771) (#1874)

Re-lands #1771, which was merged into the branch of #1768 after #1768
itself had been squash-merged and so never reached master:

- Native: the macOS addon, the Windows libmpv addon and the frame-copy
  helper observe mpv `chapter-list`; snapshots carry
  `chapters: [{ timeSeconds, title? }]`, cleared on every START_FILE.
- Main: `normalizeEmbeddedMpvChapters` validates the field into
  `EmbeddedMpvSession.chapters` (older binaries give []).
- Renderer: `buildChapterTimelineSegments` maps chapters to timeline
  segments; host catch-up segments keep precedence, and a closing-credits
  chapter times the Up next card.

Chapters flow through the same timelineSegments path as catch-up
programmes, so they get #1854's keyboard and screen-reader naming,
bounded label and 3:1 separators; specs and the frame-copy E2E now
assert the chapter name reaches the slider's aria-valuetext. The packaged
Linux smoke registers each media server for cleanup as it starts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/embedded-mpv-session.interface.ts

@@ -91,6 +91,13 @@ export interface EmbeddedMpvAudioTrack {
 
 export type EmbeddedMpvSubtitleTrack = EmbeddedMpvAudioTrack;
 
+/** One entry of mpv's `chapter-list`, in file order. */
+export interface EmbeddedMpvChapter {
+    /** Chapter start, in media seconds. */
+    timeSeconds: number;
+    title?: string;
+}
+
 export interface EmbeddedMpvRecordingState {
     active: boolean;
     targetPath?: string;
@@ -149,6 +156,11 @@ export interface EmbeddedMpvSession {
     selectedSubtitleTrackId: number | null;
     playbackSpeed: number;
     aspectOverride: string;
+    /**
+     * The file's chapters (mpv `chapter-list`). Absent from an addon built
+     * before chapters were observed and from the Linux `--wid` backend.
+     */
+    chapters?: EmbeddedMpvChapter[];
     /** Source video size (mpv dwidth/dheight); frame-copy engine only. */
     videoWidth?: number;
     videoHeight?: number;