perf: memoize syntax highlighting in CodeBlock

hljs.highlightAuto() ran inline in the template on every render.
Move it to a reactive statement so it only runs when code or lang changes.

Author: Algorithm5838

src/lib/components/chat/Messages/CodeBlock.svelte

@@ -53,6 +53,14 @@
 		updateCode();
 	}
 
+	$: highlightedCode = (() => {
+		try {
+			return hljs.highlightAuto(code, hljs.getLanguage(lang)?.aliases).value || code;
+		} catch {
+			return code;
+		}
+	})();
+
 	const updateCode = () => {
 		_code = code;
 	};
@@ -62,7 +70,6 @@
 	let renderHTML = null;
 	let renderError = null;
 
-	let highlightedCode = null;
 	let executing = false;
 
 	let stdout = null;
@@ -544,8 +551,7 @@
 								result) &&
 								'border-bottom-left-radius: 0px; border-bottom-right-radius: 0px;'}"><code
 								class="language-{lang} rounded-t-none whitespace-pre text-sm"
-								>{@html hljs.highlightAuto(code, hljs.getLanguage(lang)?.aliases).value ||
-									code}</code
+								>{@html highlightedCode}</code
 							></pre>
 					{/if}
 				{:else}