feat(extension): export history sessions as json

[Adonis0123]

Summary

• add a per-session export action in the extension history list
• export the selected session's HistoricalEvent[] as a JSON download
• extract filename generation and download logic into a small history export utility

Testing

• npx eslint packages/extension/src/components/HistoryList.tsx packages/extension/src/lib/history-export.ts
• npm run build:libs
• npm run build:ext -w @page-agent/ext

Closes #239

[CLAassistant]

All committers have signed the CLA.

[gaomeng1900]

Thanks for your contribution!

LGTM.