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/components/ChapterStrip.tsx

@@ -0,0 +1,87 @@
+import type { Chapter } from '../types'
+import {
+  activeChapterIndex,
+  chapterKindLabel,
+  chapterLapRange,
+  chapterStartScrub,
+} from '../lib/chapters'
+import '../styles/chapters.css'
+
+interface Props {
+  chapters: Chapter[]
+  scrubTime: number | null
+  tMin: number
+  tRange: number
+  tourActive: boolean
+  tourChapterIndex: number | null
+  onChapterClick: (index: number, scrub: number) => void
+  onTourToggle: () => void
+}
+
+export function ChapterStrip({
+  chapters,
+  scrubTime,
+  tMin,
+  tRange,
+  tourActive,
+  tourChapterIndex,
+  onChapterClick,
+  onTourToggle,
+}: Props) {
+  if (chapters.length === 0) {
+    return (
+      <div className="chapter-strip" data-testid="chapter-strip">
+        <p className="chapter-strip-empty">No story chapters for this session.</p>
+      </div>
+    )
+  }
+
+  const activeIndex = activeChapterIndex(chapters, scrubTime, tMin, tRange)
+
+  return (
+    <div className="chapter-strip" data-testid="chapter-strip">
+      <div className="chapter-strip-header">
+        <span className="chapter-strip-title">Race chapters</span>
+        <div className="chapter-strip-actions">
+          <button
+            type="button"
+            className={`chapter-tour-btn ${tourActive ? 'active' : ''}`}
+            onClick={onTourToggle}
+            aria-pressed={tourActive}
+          >
+            {tourActive ? 'Exit 90s' : '90s tour'}
+          </button>
+        </div>
+      </div>
+      <div className="chapter-strip-scroll" role="list" aria-label="Race story chapters">
+        {chapters.map((chapter, index) => {
+          const scrub = chapterStartScrub(chapter, tMin, tRange) ?? index / Math.max(chapters.length - 1, 1)
+          const isActive = tourActive
+            ? tourChapterIndex === index
+            : activeIndex === index
+          const headline = chapter.headline || chapter.title
+
+          return (
+            <button
+              key={`${chapter.kind}-${chapter.start_lap}-${index}`}
+              type="button"
+              role="listitem"
+              className={`chapter-card ${isActive ? (tourActive ? 'tour-active' : 'active') : ''}`}
+              onClick={() => onChapterClick(index, scrub)}
+              aria-current={isActive ? 'true' : undefined}
+              data-testid={`chapter-card-${index}`}
+            >
+              <div className="chapter-card-top">
+                <span className={`chapter-kind chapter-kind--${chapter.kind}`}>
+                  {chapterKindLabel(chapter.kind)}
+                </span>
+                <span className="chapter-lap-range">{chapterLapRange(chapter)}</span>
+              </div>
+              <span className="chapter-headline">{headline}</span>
+            </button>
+          )
+        })}
+      </div>
+    </div>
+  )
+}