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/test/chapters.test.ts

@@ -0,0 +1,63 @@
+import { describe, expect, it } from 'vitest'
+import {
+  activeChapterIndex,
+  chapterKindLabel,
+  chapterLapRange,
+  chapterStartScrub,
+  chapterTourDurations,
+} from '../lib/chapters'
+import type { Chapter } from '../types'
+
+const sampleChapters: Chapter[] = [
+  {
+    kind: 'start',
+    title: 'Start',
+    headline: 'Lights out — the field charges into Turn 1',
+    start_lap: 1,
+    end_lap: 1,
+    start_time: '2025-05-25T13:00:00Z',
+    end_time: '2025-05-25T13:01:00Z',
+    driver_numbers: [],
+  },
+  {
+    kind: 'safety_car',
+    title: 'Safety Car (L12-L15)',
+    headline: 'Sainz incident brings out the Safety Car — leaders dive for the pits',
+    start_lap: 12,
+    end_lap: 15,
+    start_time: '2025-05-25T13:12:00Z',
+    end_time: '2025-05-25T13:15:00Z',
+    driver_numbers: [55],
+  },
+]
+
+describe('chapters lib', () => {
+  it('labels chapter kinds', () => {
+    expect(chapterKindLabel('safety_car')).toBe('SC')
+    expect(chapterKindLabel('finish')).toBe('FIN')
+  })
+
+  it('formats lap ranges', () => {
+    expect(chapterLapRange(sampleChapters[0])).toBe('L1')
+    expect(chapterLapRange(sampleChapters[1])).toBe('L12–L15')
+  })
+
+  it('maps chapter start time to scrub position', () => {
+    const tMin = new Date('2025-05-25T13:00:00Z').getTime()
+    const tMax = new Date('2025-05-25T13:20:00Z').getTime()
+    const scrub = chapterStartScrub(sampleChapters[1], tMin, tMax - tMin)
+    expect(scrub).toBeCloseTo(0.6, 2)
+  })
+
+  it('finds active chapter from scrub time', () => {
+    const tMin = new Date('2025-05-25T13:00:00Z').getTime()
+    const tMax = new Date('2025-05-25T13:20:00Z').getTime()
+    const tRange = tMax - tMin
+    const scrub = (new Date('2025-05-25T13:13:00Z').getTime() - tMin) / tRange
+    expect(activeChapterIndex(sampleChapters, scrub, tMin, tRange)).toBe(1)
+  })
+
+  it('splits 90s evenly across chapters', () => {
+    expect(chapterTourDurations(sampleChapters)).toEqual([45_000, 45_000])
+  })
+})