feat: add narrative chapter headlines and race-hub chapter strip (#21)

Generate deterministic template headlines server-side for each replay chapter
kind, expose Chapter.headline in the race-hub payload, and render a horizontal
chapter strip on the story view with active-chapter highlighting, click-to-jump,
and a 90-second tour mode built on the existing scrubber playback.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/lib/chapters.ts

@@ -0,0 +1,82 @@
+import type { Chapter } from '../types'
+
+export function chapterKindLabel(kind: string): string {
+  switch (kind) {
+    case 'start':
+      return 'GO'
+    case 'safety_car':
+      return 'SC'
+    case 'virtual_safety_car':
+      return 'VSC'
+    case 'red_flag':
+      return 'RF'
+    case 'pit_phase':
+      return 'PIT'
+    case 'decisive_swing':
+      return '▲'
+    case 'finish':
+      return 'FIN'
+    default:
+      return kind.slice(0, 3).toUpperCase()
+  }
+}
+
+export function chapterLapRange(chapter: Chapter): string {
+  if (chapter.start_lap === chapter.end_lap) {
+    return `L${chapter.start_lap}`
+  }
+  return `L${chapter.start_lap}–L${chapter.end_lap}`
+}
+
+/** Normalized scrub position (0–1) for a chapter's start time on the chart axis. */
+export function chapterStartScrub(
+  chapter: Chapter,
+  tMin: number,
+  tRange: number,
+): number | null {
+  if (!chapter.start_time || tRange <= 0) return null
+  const ms = new Date(chapter.start_time).getTime()
+  if (Number.isNaN(ms)) return null
+  return Math.max(0, Math.min(1, (ms - tMin) / tRange))
+}
+
+/** Normalized scrub position (0–1) for a chapter's end time on the chart axis. */
+export function chapterEndScrub(
+  chapter: Chapter,
+  tMin: number,
+  tRange: number,
+): number | null {
+  const raw = chapter.end_time ?? chapter.start_time
+  if (!raw || tRange <= 0) return null
+  const ms = new Date(raw).getTime()
+  if (Number.isNaN(ms)) return null
+  return Math.max(0, Math.min(1, (ms - tMin) / tRange))
+}
+
+/** Index of the chapter containing the current scrub position, if any. */
+export function activeChapterIndex(
+  chapters: Chapter[],
+  scrubTime: number | null,
+  tMin: number,
+  tRange: number,
+): number | null {
+  if (scrubTime === null || chapters.length === 0 || tRange <= 0) return null
+  const chartMs = tMin + scrubTime * tRange
+  for (let i = 0; i < chapters.length; i++) {
+    const ch = chapters[i]
+    const startMs = ch.start_time ? new Date(ch.start_time).getTime() : NaN
+    const endRaw = ch.end_time ?? ch.start_time
+    const endMs = endRaw ? new Date(endRaw).getTime() : NaN
+    if (!Number.isNaN(startMs) && !Number.isNaN(endMs) && chartMs >= startMs && chartMs <= endMs) {
+      return i
+    }
+  }
+  return null
+}
+
+/** Duration in ms each chapter should play during a ~90s tour. */
+export function chapterTourDurations(chapters: Chapter[], totalMs = 90_000): number[] {
+  if (chapters.length === 0) return []
+  const perChapter = totalMs / chapters.length
+  return chapters.map(() => perChapter)
+}