fix(ui): add KaTeX CSS and normalize LaTeX math delimiters for proper formula rendering

Import KaTeX CSS in layout.tsx to ensure math formulas display correctly
in both dev and production modes. Convert LaTeX-style `\[...\]` / `\(...\)`
delimiters to `$$...$$` / `$...$` since remark-math only supports dollar
sign delimiters, fixing formula rendering for agents like Codex.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/layout.tsx

@@ -1,4 +1,5 @@
 import type { Metadata, Viewport } from "next"
+import "katex/dist/katex.min.css"
 import "./globals.css"
 import { JetBrains_Mono } from "next/font/google"
 import { NextIntlClientProvider } from "next-intl"