Improve PNG export quality

[haan]

Summary

• Export PNGs from a dedicated offscreen React Flow renderer instead of the current user viewport.
• Compute full active-view bounds for visible diagram content and annotations.
• Use capped high-resolution output and preserve the existing white background and export exclusions.
• Fix class accent rendering for PNG export and wait for offscreen flow initialization before capture.

Validation

• npm run lint
• npm run test:unit
• npm run build:ci