feat(chapters): add pure helpers and unit tests

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.

utils/chapters.ts

@@ -0,0 +1,90 @@
+/**
+ * Pure helpers for Jellyfin chapter markers. Dependency-free so they are
+ * unit-testable under `bun test`.
+ */
+
+import type { ChapterInfo } from "@jellyfin/sdk/lib/generated-client/models";
+import { ticksToMs } from "@/utils/time";
+
+export interface ChapterMarker {
+  /** Chapter start, in milliseconds. */
+  positionMs: number;
+  /** Chapter start as a percentage (0-100) of the media duration. */
+  percent: number;
+}
+
+export interface ChapterEntry {
+  chapter: ChapterInfo;
+  /** Chapter start, in milliseconds. */
+  positionMs: number;
+}
+
+/** Chapters paired with their millisecond start, sorted ascending by start. */
+export const sortedChapters = (
+  chapters: ChapterInfo[] | null | undefined,
+): ChapterEntry[] =>
+  (chapters ?? [])
+    .filter((c) => c.StartPositionTicks != null)
+    .map((chapter) => ({
+      chapter,
+      positionMs: ticksToMs(chapter.StartPositionTicks),
+    }))
+    .sort((a, b) => a.positionMs - b.positionMs);
+
+/** Chapter start positions in milliseconds, ascending. */
+export const chapterStartsMs = (
+  chapters: ChapterInfo[] | null | undefined,
+): number[] =>
+  (chapters ?? [])
+    .filter((c) => c.StartPositionTicks != null)
+    .map((c) => ticksToMs(c.StartPositionTicks))
+    .sort((a, b) => a - b);
+
+/** Chapter markers within [0, durationMs]; empty when duration is unknown. */
+export const chapterMarkers = (
+  chapters: ChapterInfo[] | null | undefined,
+  durationMs: number,
+): ChapterMarker[] => {
+  if (durationMs <= 0) return [];
+  return chapterStartsMs(chapters)
+    .filter((ms) => ms >= 0 && ms < durationMs)
+    .map((ms) => ({ positionMs: ms, percent: (ms / durationMs) * 100 }));
+};
+
+/** Index of the chapter containing `positionMs`, or -1 if before the first. */
+export const currentChapterIndex = (
+  positionMs: number,
+  chapters: ChapterInfo[] | null | undefined,
+): number => {
+  const starts = chapterStartsMs(chapters);
+  let index = -1;
+  for (let i = 0; i < starts.length; i++) {
+    if (positionMs >= starts[i]) index = i;
+    else break;
+  }
+  return index;
+};
+
+/** Name of the chapter containing `positionMs`, or null if none / unnamed. */
+export const chapterNameAt = (
+  positionMs: number,
+  chapters: ChapterInfo[] | null | undefined,
+): string | null => {
+  const idx = currentChapterIndex(positionMs, chapters);
+  if (idx < 0) return null;
+  const sorted = sortedChapters(chapters);
+  const name = sorted[idx]?.chapter.Name;
+  return name && name.length > 0 ? name : null;
+};
+
+/** `m:ss` (or `h:mm:ss` past an hour) label for a millisecond position. */
+export const formatChapterTime = (positionMs: number): string => {
+  const total = Math.max(0, Math.floor(positionMs / 1000));
+  const hours = Math.floor(total / 3600);
+  const minutes = Math.floor((total % 3600) / 60);
+  const seconds = total % 60;
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return hours > 0
+    ? `${hours}:${pad(minutes)}:${pad(seconds)}`
+    : `${minutes}:${pad(seconds)}`;
+};