TimeSeriesChart: forward chain prop to /api/series so chain tab switch re-fetches

[Flotapponnier]

Bug: on a bench with dimensions.chain (aggregator-head-lag, metadata-coverage, …), clicking a different chain tab keeps the 7d/30d sparkline stuck on whichever chain was loaded first. The 24h sparkline updates (server-rendered per chain via viewBenchmark) but the lazy-fetched longer ranges don't.

Root cause in src/components/time-series-chart/index.tsx:

• buildQs only forwarded region, never chain.
• useEffect deps were [regionProp, benchmark.slug] — no chain — so a chain change triggered no re-fetch.

The /api/series/[slug] endpoint already accepts and honors ?chain= (route.ts:130 iterates ["chain","region","kind","venue"]), so the fix is purely client-side wiring.

Change

1. TimeSeriesChart gains chain?: string prop mirroring region?: string.
2. buildQs sets chain when the prop is set and not "all".
3. useEffect deps include chainProp so a chain tab flip triggers the re-fetch.
4. BenchmarkBody passes chain={effectiveChain ?? undefined} down.

Test plan

• bun run typecheck — clean
• After deploy: on aggregator-head-lag click Base → BNB → Solana tabs, the 7d/30d chart should refresh with each click (before: stuck on first-loaded chain)