Render LaTeX math in assistant messages

[Daniel-Valembois]

Problem
Math in assistant replies shows up as raw LaTeX ($\sum_{n=1}^{\infty}$, $$...$$), which makes T3 hard to use for maths and technical study. The Claude app and Claude Code both render it.

Solution
Render inline $...$ and display $$...$$ math with KaTeX in the message view, on desktop and mobile.

[juliusmarminge]

Duplicate of #9641 — same ask to render inline $...$ / \(...\) and display $$...$$ / \[...\] math (KaTeX) in assistant messages.

Tracking / upvote there. Open fix PR: #9838. Related history: #4585, #1784, #8430; earlier attempts #7166, #7739.