fix: rendering for model-emitted bracketed math blocks

[baisechundu]

Summary

Fix rendering for model-emitted display math blocks written as standalone bracketed lines.

Some models emit formulas like this:

[ C(x) = \frac{2}{T(T-1)} \sum_{i<j} S(\hat{y}^{(i)}, \hat{y}^{(j)}) ]

Pi Web previously rendered that as plain text. This change normalizes likely math-only bracket lines into standard display
math before Markdown parsing:

$$
C(x) = \frac{2}{T(T-1)} \sum_{i<j} S(\hat{y}^{(i)}, \hat{y}^{(j)})
$$

Expected rendered result:

$$C(x) = \frac{2}{T(T-1)} \sum_{i<j} S(\hat{y}^{(i)}, \hat{y}^{(j)})$$

Changes

• Normalize standalone [ ... ] lines that look like math into display math fences.
• Leave ordinary bracket-only prose unchanged.
• Add regression tests for normalization and KaTeX rendering.

Test Plan

node --experimental-strip-types --test components/MarkdownBody.test.mjs lib/markdown.test.mjs
node_modules/.bin/tsc --noEmit
git diff --check -- lib/markdown.ts components/MarkdownBody.test.mjs lib/markdown.test.mjs