fix: re-typeset LaTeX content after any update

[pyramation]

Summary

A client-side navigation on a page that already rendered math un-rendered it: equations reverted to raw $$…$$ source. Reproduced on LaTeX2JS.com by fresh-loading the homepage and clicking the "LaTeX2JS" home link — mjx-container count dropped 31 → 13 and the raw source came back.

Two things combined:

• render() built new LaTeX2HTML5() and re-parsed on every render, so an unrelated re-render produced brand-new element objects; React tore down and rebuilt the dangerouslySetInnerHTML nodes, discarding MathJax's typeset output and restoring raw source.
• componentDidUpdate only typeset when the content prop changed, so nothing put the math back:

-componentDidUpdate(prevProps) {
-  if (prevProps.content !== this.props.content && this.state.mathJaxLoaded) {
+componentDidUpdate() {
+  if (this.state.mathJaxLoaded) {
     this.typesetMath();

The parse and the resulting child elements are now cached on the instance keyed by content, so an unchanged content re-render reuses the same elements and the DOM is left alone; caching the parse alone was not enough — PSTricks children still churned. Typesetting on every update is the safety net for the cases where the markup does get rewritten (MathJax skips math it has already typeset). The mount path drops its explicit typesetMath() because the mathJaxLoaded setState now flows through componentDidUpdate.

Verified in a browser against LaTeX2JS.com built with this package: 31 mjx-container / 0 raw $$ on fresh load, unchanged after three consecutive home-link navigations.

Link to Devin session: https://app.devin.ai/sessions/e51e8ecb94d241bd8d89e65e642c3e43
Requested by: @pyramation

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