#1830 Add locally bundled KaTeX rendering to web chat

Descripción

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

Replaces the placeholder adapter with KaTeX 0.19.0. 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 JavaScript, CSS, WOFF2 fonts, license, and provenance: **558,806 bytes** (about 0.56 MB). No CDN or TeX installation is required. Uses HTML plus semantic MathML, untrusted input, isolated macros, bounded expansion, and bounded requested sizes.

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.

This is the lightweight first renderer. MathJax and full TeX remain separate alternatives. [Adapter-only diff](https://github.com/CC-David-CC/Strata-a5500/compare/feat/web-math-common...feat/web-math-katex). Usage, reproduction commands, and size/feature comparisons are in docs/WEB_MATH.md and docs/WEB_MATH_COMPARISON.md.