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/ReplayTrackMap.test.tsx

@@ -44,9 +44,10 @@ const results: EnrichedResult[] = [
 ]
 
 describe('ReplayTrackMap', () => {
-  it('renders an empty state when replay frames are missing', () => {
+  it('renders an empty-state card when replay frames are missing', () => {
     render(<ReplayTrackMap outline={outline} replay={{ ...replay, frames: [] }} tMs={0} drivers={[]} results={results} />)
-    expect(screen.getByTestId('replay-track-map')).toHaveTextContent(/historical GPS unavailable/i)
+    expect(screen.getByTestId('replay-map-no-frames')).toBeInTheDocument()
+    expect(screen.getByText('Historical GPS unavailable')).toBeInTheDocument()
   })
 
   it('renders car labels from result metadata', () => {