Add MathJax Support for LaTeX Equations

**Description:**

- Add MathJax rendering for LaTeX expressions in Markdown (like $$x^2+y^2=z^2$$).
- Load MathJax script.
- After parsing markdown, call MathJax typeset to render equations.
- Test inline $E=mc^2$ and block equations.

**Expected result:**
Users can write and render math equations easily.

No files or tests are named in the issue. Start by locating the Markdown parsing and preview-rendering entry points, then review how external scripts are loaded. Done means MathJax renders both inline and block LaTeX expressions after Markdown parsing, with the stated examples covered by tests.

Written by the indexing model from the issue text.