fix(app): a math or latex code block of equations draws as typeset math

[sean-brydon]

Asked for physics maths, an agent on cal answered with a ```latex code block, and the chat showed it as code. Math only drew from $$…$$, `(…)` and `[…]` in the reply's text, so code blocks of maths stayed code. That reply was actually a whole `physics.tex` document to compile with `pdflatex`.

Now (lib/markdown-html.ts, after micromark, through the same placeholder path as $$ blocks):

• A ```math block draws as display math. That's GitHub's syntax for a formula.
• A latex, tex or ```katex block draws as display math too, unless it looks like a document: \documentclass, `\usepackage`, `\begin{document}`, `\section`, `\title`, `\item` and the like. A document stays code, since it's meant for `pdflatex`.
• A block that hasn't closed yet, in a reply still being written, stays code until it does. A half-written formula would only show KaTeX's errors.
• Other languages and unnamed blocks are unchanged.

Proof: the app on mock data, a reply with a math block, a latex block of one equation, and a ```latex document.

Berth Light	Berth Dark

Checks

• markdown-html.test.ts: new cases cover math blocks, latex/tex equations, HTML escapes undone (<, &), documents and \section staying code, other languages untouched, and an open block in a draft.
• e2e/math.spec.ts: a new browser test passes, and the old one still does.
• tsc, check:titles, check:csp, pnpm test (441 tests), pnpm build, npx tsc -p e2e and the docs build pass.

🤖 Generated with Claude Code

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[linear-code]

ENG-6095