feat: highlight code with Zola instead of highlight.js

[magnus-madsen]

Closes #7.

The issue asked whether highlighting could move to a sublime-style grammar. It can move off highlight.js, but not to .sublime-syntax: Zola 0.22 (which this repo already runs) replaced syntect with giallo, so extra grammars are VSCode/TextMate JSON and the config moved into [markdown.highlighting]. Feeding it the Flix.sublime-syntax from flix/talks fails with a JSON parse error.

The grammar it does want already exists: flix/textmate, which flix.dev consumes the same way. Both sites now colour Flix from one grammar.

What changes

• syntaxes/flix.tmLanguage.json is vendored, byte-identical to flix/textmate at the pinned GRAMMAR_COMMIT, so a clone builds with nothing but Zola. make update-grammar refreshes it after bumping the SHA, and the result lands as a reviewable diff.
• Colours come from Catppuccin Mocha, which Zola generates into static/giallo.css. That is the family tabi's own syntax stylesheet was derived from, and its ground (#1E1E2E) and base text (#CDD6F4) sit right where tabi had them — with 381 rules behind it instead of tabi's 65.
• Deletes the 22K highlight.js bundle, its activation script, and the two hand-written stylesheets. themes/tabi/templates/partials/header.html is stock again, so tabi updates no longer need re-patching.
• Both build outputs (syntaxes/flix.tmLanguage.json, static/giallo.css) are gitignored.

What it looks like

Nine colours across the blog's Flix blocks, against four under the old setup; operators, escapes and language constants are distinguished for the first time. Scala, shell, Rust and Markdown blocks are highlighted at all for the first time — highlight_activate.js had to force nohighlight on everything except Flix, because the bundle carried only the Flix grammar.

Trade-offs worth reviewing

• Language badges change. Zola labels blocks with the resolved grammar name, so ```sh now reads SHELLSCRIPT, and ```claude, ```prompt and ```txt (15 blocks) read PLAIN. Renaming those fences is a follow-up if the labels matter.
• Function names and stdlib calls lost their colour. highlight.js guessed at them from a word list; flix/textmate has no entity.name.function or support.function rules, so no theme can recover them. Fixing that upstream would benefit flix.dev too.
• A 9KB vendored file can drift from flix/textmate until someone bumps the SHA and runs make update-grammar. The alternative — fetching at build time — was tried first and dropped: it made make mandatory, and Windows ships no make, so it would have cost Windows contributors the plain zola serve workflow they have today.
• Changing the theme needs static/giallo.css deleted by hand. Zola writes that file only when it is absent, so the old colours otherwise survive a rebuild and a running zola serve. Documented in the README rather than handled in the Makefile, which would have cost portability on every build for something done once a year.

Verified: clean-tree make builds in 0.4s, a second make is idempotent, and CI needs no change — the workflow already runs make.

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