feat(playback): report Embedded MPV file chapters on the timeline

Observe mpv `chapter-list` in the macOS and Windows addons and the
frame-copy helper, carry it as `EmbeddedMpvSession.chapters`, and map it
to `PlayerTimelineSegment[]` for `app-player-controls` when the host
passes no catch-up segments. A closing-credits chapter now times the
Up next card.

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

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

@@ -84,6 +84,13 @@ 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;
@@ -142,6 +149,11 @@ 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;