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/boot.js

@@ -584,6 +584,16 @@ function _applyTheme(name){
     ?(window.matchMedia('(prefers-color-scheme:dark)').matches?'dark':'light')
     :name;
   document.documentElement.dataset.theme=resolved||'dark';
+  // Swap Prism syntax-highlighting theme to match UI theme
+  (function(){
+    const link=document.getElementById('prism-theme');
+    if(!link) return;
+    const isDark=(resolved!=='light');
+    const want=isDark
+      ?'https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism-tomorrow.min.css'
+      :'https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism.min.css';
+    if(link.href!==want){ link.href=want; }
+  })();
   // Re-register OS change listener whenever system theme is active
   if(name==='system'){
     const mq=window.matchMedia('(prefers-color-scheme:dark)');