docs: add Flatbread brand marks

[toeknee-figma]

Describe

Replace the raster README header with a new SVG mark. Add full-color transparent and one-color light and dark SVG variants with simple usage guidance.

Test Plan

• Run pnpm verify.
• Render all four SVG variants at 16, 32, 256, and 1024 px with resvg.
• Review the primary, transparent, dark, and light variants on their intended backgrounds.

[cursor]

Approved. Cursor Bugbot was not present after the first check poll, and no approval policy requires human review for these brand-asset and README files. No reviewers were assigned.

  

_{Sent by Cursor Approval Agent: Pull Request Router and Approver}