[chphch]

Implements #1629.

Renders LaTeX math in the markdown view using KaTeX — display equations ($$…$$, \[…\]) on their own line, and inline math ($…$, \(…\)) within prose, lists, and headers. It follows the existing Mermaid special-block pattern: KaTeX runs inside a react-native-webview on native and via a lazy import('katex') on web, so a message full of equations renders as math instead of raw dollar-sign source. Currency ($5), escaped \$, and $ x $ (space after the opener) stay plain text so ordinary prose isn't mangled.

How it works

• Parser (parseMarkdownBlock.ts, parseMarkdownSpans.ts): a new math block type for display equations, and inline math split out by a single unified tokenizer so a $ inside a `code` span, a [markdown](link), or a bare URL stays literal, and TeX bodies (which contain *, _, \) aren't re-parsed as markdown. Unclosed/partial $$ (mid-stream) and trailing-text-after-close fall through to normal text handling.
• Renderer (MathView.tsx / MathView.web.tsx): MathBlock for display equations. On web, inline math is a per-span element that flows inside the normal <Text>, so native text selection is preserved. RN <Text> can't hold a WebView, so on native a text run containing inline math is rendered as one KaTeX WebView; non-math runs keep the native <Text> path.

Proof

BEFORE → AFTER on the dev/messages-demo screen (a demo agent message containing inline + display LaTeX), captured on Expo web via a throwaway standalone happy-server (PGlite) + Playwright.

BEFORE, the LaTeX shows as raw source ($\mathcal{L} = -\sum_i y_i \log \hat{y}_i$, $$ \int … $$); AFTER, inline math flows inside the prose and list items and display equations render centered on their own line. The capture also asserts Prices like $5 and $10 stays literal text (the currency guard), and that the pre-existing markdown table still renders unchanged.

Verification

pnpm typecheck clean. Parser behaviour is pinned by unit tests in parseMarkdownMath.test.ts covering block detection, inline splitting, and the currency / escaped-\$ / inline-code / link / bare-URL guards.