#1832 Add locally bundled MathJax rendering to web chat

Description

Depends on #1829 (shared LaTeX/web math core). This branch builds directly on that core; its commits remain visible against main until the core merges. Addresses #1769 and #1780.

Replaces the placeholder adapter with MathJax 4.1.3 and local New Computer Modern font assets. Enable **Render math** in Chat's sampling drawer and Apply to typeset inline and display equations. Rendering starts disabled. Code stays literal; invalid or unsupported equations retain their source. Copy, export, history, and model requests preserve the original text.

Bundles **3,204,457 bytes** (about 3.20 MB) of engine/font assets, licenses, and provenance. JavaScript, dynamic font data, and fonts load from Strata without a CDN or TeX installation. Produces CommonHTML plus assistive MathML. Uses configured TeX packages, safe output options, bounded macro expansion, and per-equation macro isolation; require/autoload are disabled.

This is a separate alternative to the 0.56 MB KaTeX implementation, using the same core interface and acceptance corpus. The comparison documents syntax, size, latency, accessibility, and setup tradeoffs without claiming universal LaTeX support.

Validated in Chromium, Firefox, and Playwright WebKit on Windows, including streaming, currency/code handling, macro isolation, deadlines, stale results, source preservation, and narrow layouts. WebKit coverage is not a physical Safari/iPhone test; semantic markup checks are not a manual screen-reader audit.

[Adapter-only diff](https://github.com/CC-David-CC/Strata-a5500/compare/feat/web-math-common...feat/web-math-mathjax). Usage and reproduction commands are in docs/WEB_MATH.md; measured comparisons are in docs/WEB_MATH_COMPARISON.md.