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/templates/base.html

@@ -9,12 +9,6 @@
   <title>{{ title }} | {{ config.title }}</title>
   <link rel="canonical" href="{{ base_url }}{{ page_path }}" />
   <link rel="stylesheet" href="{{ prefix }}/style.css" />
-  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.21/dist/katex.min.css" crossorigin="anonymous"
-    media="print" onload="this.media='all'" />
-  <noscript>
-    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.21/dist/katex.min.css"
-      crossorigin="anonymous" />
-  </noscript>
   <link rel="icon" type="image/png" href="{{ prefix }}/logo.png" />
 </head>