Adds dependency-free helpers in utils/chapters.ts for working with
Jellyfin chapter markers:
- chapterMarkers(chapters, durationMs): markers within range, with
precomputed percent for slider overlays.
- chapterStartsMs(chapters): sorted start positions in ms (skips
entries without StartPositionTicks).
- currentChapterIndex(positionMs, chapters): active chapter index
for the live playback position (-1 if before the first chapter).
- chapterNameAt(positionMs, chapters): the active chapter name, or
null if missing/unnamed.
- sortedChapters(chapters): chapter entries paired with ms start.
- formatChapterTime(positionMs): m:ss or h:mm:ss label.
All helpers tolerate null/undefined/empty inputs.
17 unit tests under bun test cover sort order, boundary positions,
missing fields, and out-of-range inputs.
