Restyle site to the Academic Journal theme

[wenhaochai]

Summary

Restyles the whole site to the visual language of wenhaochai.com: warm paper, oxblood accent, hairline rules, self-hosted Newsreader / Inter Tight / JetBrains Mono, with an OS-following midnight theme (T toggles it).

• Blog index follows blogs.html: page hero + horizontal post rows (thumbnail, mono meta, serif title, description, links).
• Article pages follow the report layout of blogs/loopdit.html: crumb, date eyebrow, title, standfirst, byline, sticky Contents column with scrollspy, single body column.
• Home, Paper, Join, 404 and archive pages get the same shell; the leaderboard tabs, terminal card, example tasks and institution/sponsor grids are kept and redrawn.
• Removes the al-folio SCSS, Bootstrap/MDB/jQuery/FontAwesome and the distill.js bundle. A small assets/js/site.js replaces them (mobile nav, theme key, contents scrollspy, and shims so <d-math>, <d-cite>, <d-footnote> still render without template.v2.js).

Content is unchanged

• No file under _posts/ is touched. The body container keeps the <d-article> element so every post's own _styles block still applies.
• Built the site before and after and diffed the visible text of every page: article bodies, leaderboard numbers and links are identical. The only differences are chrome: the home cards drop the literal "Date" / "Category" labels (values kept), the terminal card no longer has the line-by-line entrance animation (text kept), and article pages gain the crumb and "All posts / Back to top" links from the report layout.

Verification

• Local Jekyll build, plus a JEKYLL_ENV=production build with jekyll-minifier: the minified CSS keeps color-mix, @supports, clamp(), the midnight theme and the article grid intact.
• Screenshots checked at desktop and mobile widths and in dark mode; MathJax $$ and <d-math> render; no console errors.

Note: filter-files will fail on this PR by design (it only admits post submissions); it has been cancelled on every recent non-post PR as well.

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