feat: add CodeMirror syntax highlighting to section page code blocks

- Use CodeMirror in read-only mode for code examples
- Auto-detect CSS vs HTML based on content
- Clean up views when navigating between sections
- Add transparent background to blend with code-block container

🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/main.css

@@ -1911,6 +1911,7 @@ input:checked + .toggle-slider::before {
 	overflow: hidden;
 }
 
+/* Fallback styles for pre/code (before CodeMirror initializes) */
 .code-block pre {
 	margin: 0;
 	padding: 1rem;
@@ -1925,6 +1926,15 @@ input:checked + .toggle-slider::before {
 	white-space: pre;
 }
 
+/* CodeMirror styles within code blocks */
+.code-block .cm-editor {
+	background: transparent;
+}
+
+.code-block .cm-scroller {
+	overflow-x: auto;
+}
+
 .code-caption {
 	font-size: 0.75rem;
 	color: var(--light-text);