[visual][arena] USD-equivalent displays: add staleness / last-updated affordance
## Summary
During local-stack visual QA ([#164](https://gitlab.com/PlasticDigits/yieldomega/-/issues/164)), **USD-equivalent** figures (e.g. total raise / podium prize hints) were present and plausible, but there was **no indication of quote age** or oracle/feed freshness. **P3 polish** — reduces silent-stale trust issues.
## What is wrong
Users see **USD** or **≈ $** amounts with **no “as of”** context, so a delayed or cached price read looks the same as a live one.
## Expected
When USD is derived from an **off-chain or secondary feed**, the UI should communicate **staleness or refresh cadence** (subtle text, tooltip, or icon), consistent with product risk appetite — without alarming users on every tick.
## Relevant docs / skills / files
- Umbrella: [#164](https://gitlab.com/PlasticDigits/yieldomega/-/issues/164) §6 item 7 (partial — staleness called out as nit).
- [`docs/frontend/timecurve-views.md`](docs/frontend/timecurve-views.md), [`docs/testing/invariants-and-business-logic.md`](docs/testing/invariants-and-business-logic.md).
- Search frontend for USD/total raise formatting and any price hooks (Arena + Simple).
## Recommended fix
1. Identify **where** USD strings are computed (single hook vs duplicated).
2. If the feed exposes **timestamp / block**, surface **“Updated …”** or a **non-blocking** stale badge after a threshold.
3. If no timestamp exists, document **implicit refresh interval** in UI copy (minimal) or add telemetry-first iteration.
## Acceptance criteria
- [ ] Any **visible USD aggregation** on `/timecurve` routes either shows **last-updated context** or documents why USD is omitted.
- [ ] No noisy banners on healthy stacks; copy matches **P3** polish bar.
## Verification checklist
- [ ] Local stack + prod-shaped env: confirm behavior when RPC/indexer is slow vs fast.
- [ ] Mobile width: staleness affordance does not wrap awkwardly in Arena/Simple headers or cards.
- [ ] Cross-link manual QA row in [`docs/testing/manual-qa-checklists.md`](docs/testing/manual-qa-checklists.md) if a repeatable visual check is added.
/cc @Brouie — tracked from [#164](https://gitlab.com/PlasticDigits/yieldomega/-/issues/164) §6.7 polish nit.
issue