fix(security): bound KaTeX rendering of remote LaTeX

maxSize 10, maxExpand 100, trust false, strict ignore, and a 5000-char
cap (rendered as plain text beyond it), so a remote sender can't blow out
layout or CPU with \rule{99999em} or deep macro expansion.

Fixes #65

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/math/KaTeX.tsx

@@ -21,12 +21,31 @@ type KaTeXProps = {
  * inline (in its error colour) rather than throwing. The HTML returned by
  * `renderToString` is produced by our own trusted call from a fixed options
  * object — it is safe to inject via `dangerouslySetInnerHTML`.
+ *
+ * `maxSize`/`maxExpand` cap how large a single glyph (`\\rule`, etc.) or macro
+ * expansion remote LaTeX can request, and `trust: false` disables commands
+ * that can embed arbitrary HTML/URLs (e.g. `\\includegraphics`, `\\href`) —
+ * without these a hostile `$$...$$` from a remote message can DoS or (via
+ * `trust`) inject unsafe links (Gitea #65). `strict: 'ignore'` keeps unknown-
+ * but-harmless LaTeX from spamming the console as before. Extremely long
+ * source is rendered as plain text rather than handed to KaTeX at all.
  */
+const MAX_LATEX_LENGTH = 5000;
+
 export default function KaTeX({ latex, displayMode = false }: KaTeXProps) {
+  if (latex.length > MAX_LATEX_LENGTH) {
+    const Plain = displayMode ? 'div' : 'span';
+    return <Plain>{latex}</Plain>;
+  }
+
   const html = katex.renderToString(latex, {
     displayMode,
     throwOnError: false,
     output: 'htmlAndMathml',
+    maxSize: 10,
+    maxExpand: 100,
+    trust: false,
+    strict: 'ignore',
   });
 
   const Wrapper = displayMode ? 'div' : 'span';
@@ -34,7 +53,7 @@ export default function KaTeX({ latex, displayMode = false }: KaTeXProps) {
   return (
     <Wrapper
       // KaTeX output is generated by our own render call (trusted-safe).
-      // eslint-disable-next-line react/no-danger
+
       dangerouslySetInnerHTML={{ __html: html }}
     />
   );