feat: render GitHub release Markdown with top notes sheet

[krwg]

Summary

• Render GitHub release bodies as GFM Markdown (tables, images, links, code) in Settings → Updates via marked + DOMPurify.
• Clicking a release opens a top sheet with the full notes; Open on GitHub still available.
• Normalize default publish repo casing to krwg/blip; Vitest coverage for table/image/sanitize paths.

Test plan

• npm test
• Settings → Updates: release cards show formatted preview (not plain excerpt)
• Click a card → top overlay with full body, tables/images if present
• Links in notes open externally; Escape / backdrop closes overlay

Closes #54

[krwg]

Notes:

• GFM via marked; HTML sanitized with DOMPurify before inject.
• Card click opens top sheet; GitHub button stops propagation.
• Preview is clamped MD; overlay shows full body.
• Default GitHub repo fallback is krwg/blip.