feat(race-story): polish chapter strip, controls, graph, and map UX (#68)

Consolidate the Race Story section into a cohesive timeline surface: custom
chapter-strip scrolling with active-card sync, unified segmented playback
controls, chapter bands on the position graph with decimated axes and
de-collided labels, map toggle gated by an on-mount replay/outline probe,
and shared empty-state cards. Adds vitest coverage and visual snapshots.

Note: replay/outline queries now probe on mount (not only when Map opens) so
the toggle can be hidden before users hit a dead panel.

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

frontend/src/test/replayMap.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, it } from 'vitest'
+import { isReplayMapAvailable } from '../lib/replayMap'
+import type { ReplayFramesResponse, TrackOutline } from '../types'
+
+const outline: TrackOutline = {
+  circuit_key: 1,
+  bounds: { minX: 0, maxX: 100, minY: 0, maxY: 100 },
+  points: [{ x: 0, y: 0 }],
+}
+
+const replay: ReplayFramesResponse = {
+  session_key: 1,
+  interval_ms: 5000,
+  start_time: '2025-05-25T13:00:00Z',
+  frames: [
+    { t: 0, cars: {} },
+    { t: 5000, cars: {} },
+  ],
+}
+
+describe('isReplayMapAvailable', () => {
+  it('returns true when frames and outline are present', () => {
+    expect(isReplayMapAvailable(replay, outline, false)).toBe(true)
+  })
+
+  it('returns false when frames are sparse', () => {
+    expect(isReplayMapAvailable({ ...replay, frames: [{ t: 0, cars: {} }] }, outline, false)).toBe(false)
+  })
+
+  it('returns false on query error', () => {
+    expect(isReplayMapAvailable(replay, outline, true)).toBe(false)
+  })
+})