fix(ui): render dollar math with marked-katex-extension

[opencode-agent]

Issue for this PR

Closes #38030

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Desktop has only rendered \(...\) inline math and $$ blocks on their own lines since #34850, so the $...$ and single-line $$...$$ that most models emit show up as raw LaTeX.

• Uses marked-katex-extension (default mode) for $...$, $$...$$, and $$ blocks, replacing our block regex. Our \(...\) inline extension stays because the package doesn't support that syntax.
• Its default rules keep prices as text: the opening $ must follow a space or line start, and the closing $ must be followed by a space or punctuation. So "$5 and $10", "$5-$10", and $HOME stay plain.
• parseSmallMarkdown used to keep text on the main-thread fast path unless it contained \( or $$. That fast path has no math support, so short messages with $x$ would never reach KaTeX. Any text with two or more $ now goes to the worker; a lone $5 stays on the fast path.
• The streaming block splitter now lexes with the same math extensions. Before, a $$ block containing a blank line was split into separately rendered blocks while streaming.

Known limits of the extension's default mode, captured in tests: tight inline math such as ($x$), $x$-axis, and CJK without spaces (因此$x^2$是) stays raw. A price and math on the same line (Pay $5 now, or $x$ later) can be read as one formula and render as a KaTeX error. \[...\] is still unsupported.

This takes a different approach from #52100.

How did you verify your code works?

• packages/ui: bun test src/context (111 pass) and bun run typecheck
• packages/session-ui: bun test src/components/markdown (88 pass) and bun run typecheck; the new stream test fails without the splitter change
• packages/app: bun run typecheck
• oxlint on the changed files: no new warnings
• Checked that inputs which backtrack badly with a naive $ regex (unclosed $ followed by thousands of backslashes) parse in about 13ms

Screenshots / recordings

Not captured in the app; verified at the parser and stream-splitter level.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Requested by: @Brendonovich (Brendan via Slack)