test(web): add deterministic Drand countdown boundary tests

[ogundeleoluwaferanmi35]

Summary

Extract the Drand countdown calculation into a pure `computeCountdown` helper and add deterministic boundary tests with fixed timestamps.

What changed

• `useDrandCountdown.ts` — Extracted `computeCountdown(targetRound, nowSecs, genesis, period)` as an exported pure function. Refactored both `localCountdown` (offline quicknet fallback) and the live chain info path inside `useDrandCountdown` to reuse it. Removed the now-unnecessary `timeOfRound` helper.
• `useDrandCountdown.test.ts` — New Node-based test file with no browser, no live Drand call, and no real timers:
  • `computeCountdown`: one second before R, exactly at R, after R, published round (secondsRemaining: 0), future round (correct targetTime + remaining seconds)
  • `formatCountdown`: zero, negative, single seconds, minutes, exactly one minute, exactly one hour, hours with minutes + seconds, large values (24h 30m 45s)
• `package.json` — Added the new test file to the `test` script.

Key design decisions

• The pure helper accepts all time inputs explicitly (`nowSecs`, `genesis`, `period`) so tests can pin the wall clock without mocking `Date.now()`.
• The existing UI, polling cadence, and `formatCountdown` signature are unchanged.

Acceptance criteria checklist

• Tests cover one second before R, exactly at R, and after R
• Published rounds report `secondsRemaining: 0`
• Future rounds report the correct target time and remaining seconds
• `formatCountdown` has boundary cases for seconds, minutes, and hours
• The new test is included in `pnpm web:test`; `web typecheck` passes

Test output

```
ℹ tests 64
ℹ pass 64
ℹ fail 0
ℹ duration_ms 6719.737373
```

Typecheck

`pnpm --filter @sub-rosa/web typecheck` passes cleanly.

Closes #174

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