fix(themes): swap Prism syntax-highlighting theme on light/dark switch

The Prism CSS was hardcoded to prism-tomorrow (dark-only), so code
blocks stayed dark even when switching to Light or other non-dark themes.

- Add id='prism-theme' to the <link> element for runtime lookup
- In _applyTheme(), swap href between prism-tomorrow (dark) and
  prism (light) based on resolved theme
- Skips DOM write when the target href is already active

Fixes #505

static/index.html

@@ -9,7 +9,7 @@
   <!-- KaTeX math rendering CSS (loaded eagerly to prevent layout shift) -->
   <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/katex.min.css" integrity="sha384-5TcZemv2l/9On385z///+d7MSYlvIEw9FuZTIdZ14vJLqWphw7e7ZPuOiCHJcFCP" crossorigin="anonymous">
   <!-- Prism.js syntax highlighting (loaded async, non-blocking) -->
-  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism-tomorrow.min.css" integrity="sha384-wFjoQjtV1y5jVHbt0p35Ui8aV8GVpEZkyF99OXWqP/eNJDU93D3Ugxkoyh6Y2I4A" crossorigin="anonymous">
+  <link id="prism-theme" rel="stylesheet" href="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism-tomorrow.min.css" integrity="sha384-wFjoQjtV1y5jVHbt0p35Ui8aV8GVpEZkyF99OXWqP/eNJDU93D3Ugxkoyh6Y2I4A" crossorigin="anonymous">
   <script src="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/components/prism-core.min.js" integrity="sha384-MXybTpajaBV0AkcBaCPT4KIvo0FzoCiWXgcihYsw4FUkEz0Pv3JGV6tk2G8vJtDc" crossorigin="anonymous" defer></script>
   <script src="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/plugins/autoloader/prism-autoloader.min.js" integrity="sha384-Uq05+JLko69eOiPr39ta9bh7kld5PKZoU+fF7g0EXTAriEollhZ+DrN8Q/Oi8J2Q" crossorigin="anonymous" defer></script>
 </head>