refactor(math): use native MathML instead of KaTeX CDN

Remove external KaTeX stylesheet dependency in favor of browser-native
MathML rendering. Aligns with suckless philosophy—zero external deps,
works offline, smaller page loads.

- Remove KaTeX CDN links from base.html
- Hide katex-html layer, let browsers render MathML natively
- Clean up obsolete CSS (.hide-tail, .katex-display)
- Add minimal .math-display for block math layout

docs/static/style.css

@@ -262,22 +262,19 @@ a:hover {
   color: var(--accent-hover);
 }
 
-/* KaTeX display math - local overrides */
-.katex-display {
+/* Math (native MathML rendering) */
+.math-display {
+  display: block;
   margin: 1rem 0;
   overflow-x: auto;
+  text-align: center;
 }
 
-/* Hide katex-html (requires CDN fonts), use native MathML rendering instead */
+/* Hide KaTeX HTML layer, use browser's native MathML instead */
 .katex-html {
   display: none;
 }
 
-/* Fallback: constrain radical SVG when CDN blocked */
-.hide-tail svg {
-  max-width: 100% !important;
-}
-
 /* Code */
 code {
   font-family: