Redesign the site with a dark atmospheric theme

[Rochet2]

Summary

A full visual redesign of the site into a dark, atmospheric "Nightforge" design system while preserving all content, Jekyll structure, Liquid logic, and information architecture.

• Design system — new tokens.css (OKLCH palette: near-black canvas, WoW-gold accent, arcane-violet secondary) and design.md documenting the locked system. Type pairing: Space Grotesk (display) / Inter (body) / JetBrains Mono (tags & code), loaded via Google Fonts.
• Layout — _layouts/default.html now has a floating-pill nav (blur backdrop) and an Ft5 statement footer; fixed gold + violet radial blooms on the dark canvas.
• Home — compact marquee hero.
• Downloads — the catalogue is now a responsive card grid (icon + title + blurb from each post's description + tags from projects.yml).
• Project pages — dark-themed markdown body, project header, and download links restyled as mono chips.
• Gallery — js/gallery.js gains prev/next arrows and dot controls, pauses on hover/focus, and respects prefers-reduced-motion.
• 404 / Contact — restyled to match.
• Cleanup — removed the now-unused css/github_markdown.css (its base rules folded into style.css); updated script/asset_version.py and the README to track tokens.css instead.

The new linked asset tokens.css carries the ?v={{ assets_version }} cache-bust param and is included in asset_version.py, so CI's asset-version bump will cover it. _data/assets.yml is intentionally left untouched for CI to update on this PR.

Note: .hallmark/log.json is a small design-system bookkeeping file — drop it from the PR if you'd rather not track it.

Test plan

• Verify the GitHub Pages build succeeds and CI updates _data/assets.yml.
• Check home, downloads, a project post, contact, and a 404 render correctly with the new theme.
• Confirm the gallery autoplay/prev/next/dots work and pause on hover.
• Confirm fonts load and there is no flash of unstyled content.
• Sanity-check mobile widths (320 / 375 / 768 px).