Frontend: Unified Arena page — multi-timer hero, buy hub, four podiums

Context

Unified TimeArena frontend replacing Simple/Arena split. Parent epic: #238 (closed).

Relevant files

• Merge TimeCurveSimplePage.tsx + TimeCurveArenaView.tsx → TimeArenaPage.tsx
• frontend/src/pages/timecurve/TimeCurveTimerHero.tsx — multi-timer variant
• frontend/src/pages/timecurve/TimeCurveSimplePodiumSection.tsx
• Routes: /arena primary; redirect /timecurve

Acceptance criteria

• Hero shows Last Buy primary countdown + chips for Streak / Time Booster / WarBow deadlines
• Buy panel: DOUB-primary; pay toggles DOUB / CL8Y / ETH / USDm / CRED (70)
• Four podium cards with epoch id + live rankings from indexer
• WarBow hero actions with DOUB cost labels
• Remove Simple/Arena/Protocol split or reduce Protocol to admin/audit only

Verification checklist

• Manual QA: multi-timer display matches RPC
• Playwright smoke: buy + podium refresh (e2e/anvil-*.spec.ts)