[codex] Cap and dedupe toast queue

[vku2018]

Summary

• Cap the visible toast queue at three items and clear timers for toasts dropped from the queue.
• Collapse duplicate messages with the same level into one toast with a count badge, refreshing the auto-dismiss timer instead of stacking repeats.
• Preserve info status / error alert roles, the polite live region, and document the queue behavior in README.

Refs #108

Validation

• npm ci --ignore-scripts --no-audit
• npm test -- src/components/tests/ToastProvider.test.tsx (5 passing)
• npm run lint
• git diff --check (passes with existing CRLF warnings only)
• npm run build (blocked by pre-existing src/app/quote/Client.tsx missing assetsDiffer)
• npm test (109 passing, 10 failing in pre-existing quote page tests due the same missing assetsDiffer)