feat: roulette spinner auto-select on the results page

[MichaelJShepherd]

Summary

Implements TODO task 12 (roulette / spinner selection) as an optional "Can't decide? Spin for me 🎲" control on the results page, rather than a separate spinner screen. Manual selection remains the primary path.

• The highlight travels across the existing 3–5 result cards with a decelerating step sequence (spin-plan.ts, ease-in cubic delays), landing on a random pick in ~2–3s — inside the brand kit's 2–4s spinner window — and pulses the active card with the previously unused book-spin keyframe.
• On landing, an aria-live panel announces the chosen book with This is the one! (reuses the existing selection tracking + confirmation flow) and Spin again.
• prefers-reduced-motion skips the animation entirely and reveals the result instantly.
• The spin control only appears when there is more than one pick; cards are disabled mid-spin.
• TODO.md task 12 moved to Done; the spec's as-built section updated accordingly.

Screenshots

Mid-spin	Landed (accept / respin)	Accepted

Reduced motion — instant reveal (7ms, no roulette animation)

Screenshots are hosted on the chore/pr-12-screenshots branch (not for merging); delete it once this PR is closed if you don't mind the images going blank here.

Test plan

• 7 new unit tests for the spin schedule (spin-plan.spec.ts): lands on target, advances one card at a time, delays never decrease, total duration stays within 2–4s for 2–5 picks — all 21 frontend tests pass
• Production build clean
• Verified in a real browser (Puppeteer): full import→quiz→results flow, spin resolves in ~2.6s to a single book, respin works, accept reaches the confirmation screen, and reduced-motion emulation reveals instantly

https://claude.ai/code/session_01T3Bf7bT36EKqybyL4X2uKQ