feat(gui): render LaTeX math in chat via KaTeX (#208)

[justrach]

Problem

Agent responses emit LaTeX, but the dependency-free chat Markdown parser had no math node. Display formulas and inline math rendered as raw source:

\[
G \approx 1+\alpha+\alpha^2+\cdots+\alpha^m.
\]

The user saw backslashes, commands, and delimiters instead of typeset notation.

Fix

Parser (types.ts, parser.ts) — adds an inline and a display math AST node and recognizes all four agent-emitted forms:

form	example	level
\[...\]	multi-line display	block
$$...$$	display	block
\(...\)	inline	inline
$...$	inline	inline

Guardrails, all covered by tests:

• Inline code keeps top precedence, so a $ inside backticks stays code; fences are captured before math, so $$ inside a fenced block stays code.
• The $...$ form is deliberately narrow (opener/closer hug the content, never against a digit, never after a backslash), so prose currency like $5 and $10 and an escaped \$ are left as text.
• An unclosed delimiter (still streaming) degrades to open math / literal text instead of throwing or swallowing the rest of the message.

Renderer (KatexMath.tsx, MarkdownRenderer.tsx) — a new component typesets the captured LaTeX with KaTeX:

• renderToString is pure (no DOM), so it stays server-renderable and unit-testable.
• throwOnError: false → malformed/streaming input renders as a visible KaTeX error rather than throwing.
• trust: false → \href/raw-HTML commands can't inject anything unsafe (verified: no anchor, no javascript: href, raw <img onerror> escaped).
• katex.min.css is bundled via the existing @import block in styles/index.css (mirroring shadcn/tailwind.css), so the component carries no CSS import.

Tests

• parser.test.ts +7: display/inline parsing, code + fence protection, escaped $, prose currency, streaming degradation.
• MarkdownRenderer.test.tsx +4 (full pipeline via renderToStaticMarkup): display + inline typeset to KaTeX, $ in code stays code, unsafe \href emits no anchor / javascript: href.
• gui bun test: 146/146; eslint clean; new files typecheck clean under strict tsc.

Honest note on verification

The full Tauri/Vite build (bun run build) is not exercised in my environment (its gen:desktop-contracts step needs cargo), so the bundled-CSS visual result wasn't run here. The HTML pipeline is fully unit-tested and the CSS @import follows the repo's existing pattern; a quick visual sanity check on a real build is worthwhile before merge.

Dependency

Adds katex (~0.17) + @types/katex. Follows the existing library-backed-render precedent (MermaidDiagram).

Closes #208