feat(site): showcase themes and social sharing

[fcoury]

The editor gallery currently understates Red’s bundled theme range, and the existing social card does not capture the impact of the live product. This change strengthens both the on-site preview story and how Red appears when shared.

• Recapture Edit in Kanso Ink, Files in GitHub Light, Commands in Tokyo Night Storm, Agent in Rosé Pine Dawn, and Git review in Cyberdream
• Preserve the 1208×704 capture dimensions and real Red/Ghostty UI states
• Surface theme names in preview notes and accessible descriptions
• Replace the Open Graph image with a high-impact 1200×630 Red/editor/agent-review composition
• Cache-bust the social image URL and align Open Graph and X copy around reviewable agent proposals
• Assert the theme set, social metadata, image budget, and capture dimensions in rendered-site tests

How to Test

1. Open the homepage, scroll to the editor gallery, and click Edit through Git review. Each tab should show its named theme and the expected editor state.
2. Inspect the page metadata and open /og.png?v=2. The Open Graph and X image should resolve to the new 1200×630 card with exact Red copy.
3. Navigate the gallery with arrow keys and resize to a narrow viewport. The images should stay contained, and the unchanged Welcome tab should still work.
4. Run npm test and npm run lint; both should pass.