feat: add GitHub Pages deployment

[superposition]

What changed

• add a Rust static exporter that renders the canonical coldfront-ui landing page
• emit Pages-safe relative assets, .nojekyll, and a fallback 404.html
• add the GitHub Pages build and deployment workflow
• document local preview and optional Clerk configuration

Why

Cold Front currently requires the Topcoat development server. This gives the project a static artifact that can be reviewed locally and hosted on GitHub Pages without coupling the production page to Coatcheck.

Impact

After this merges and GitHub Pages is configured to use GitHub Actions, pushes to main publish the site. Authentication remains optional through the CLERK_PUBLISHABLE_KEY repository variable.

Validation

• ./scripts/check
• ./scripts/export-pages
• local HTTP smoke: page, CSS, JavaScript, and 404.html returned 200
• node --check crates/coldfront-ui/src/auth.js
• git diff --check