test: lock document workspace scrolling

[ShenzheZhu]

Closes #999

Summary

• add a browser-local Playwright workspace fixture with a long document and multi-page PDF
• assert the page itself never scrolls while PDF and editor panes scroll independently at 1440x900 and 1024x768
• keep navbar, editor toolbar, and PDF controls visible after internal scrolling
• treat demo document IDs as editor routes so the transition wrapper cannot break full-height workspace layout
• run the regression automatically in CI

Verification

• pnpm test:e2e:workspace
• pnpm test:runnable
• pnpm test:audit
• pnpm script:audit
• pnpm type-check
• pnpm lint
• pnpm build:all