Add Mastra-style workflow run studio

[falfaddaghi]

Summary

• add a Mastra-inspired workflow run studio with live graph playback, run history, nested graph drill-in, and richer node inspection
• add a demo workflow loader plus clear-history controls to make the studio testable from the UI
• add Playwright coverage and config for the studio flow while excluding generated output artifacts from version control

Testing

• npm run typecheck
• npm run build
• npm run test:e2e