feat(projects): render descriptions with Markdown

[HazAT]

Summary

• render project descriptions with react-markdown and GitHub-flavored Markdown
• preserve existing single-line breaks and safely handle external, fragment, and raw-HTML content
• provide compact, clamped Markdown previews on project and voting cards
• add form guidance and responsive styling for headings, lists, code, tables, task lists, and images
• add regression coverage for rendering and unsafe input sanitization

Verification

• npm run verify
• Claude Fable 5 code review: approved

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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