perf: move marked extension registration to module-level and remove unnecessary async wrapper

- Move marked.use() calls to <script context="module"> so extensions are
  registered once per module load instead of once per component instance,
  preventing duplicate extension stacking on the global marked singleton
- Remove unnecessary async IIFE around the reactive lexer statement since
  marked.lexer() is synchronous, eliminating needless Promise/microtask overhead

Author: Algorithm5838

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

@@ -1,17 +1,34 @@
-<script>
+<script context="module">
 	import { marked } from 'marked';
-	import { replaceTokens, processResponseContent } from '$lib/utils';
-	import { user } from '$lib/stores';
 
 	import markedExtension from '$lib/utils/marked/extension';
 	import markedKatexExtension from '$lib/utils/marked/katex-extension';
 	import { disableSingleTilde } from '$lib/utils/marked/strikethrough-extension';
 	import { mentionExtension } from '$lib/utils/marked/mention-extension';
-
-	import MarkdownTokens from './Markdown/MarkdownTokens.svelte';
 	import footnoteExtension from '$lib/utils/marked/footnote-extension';
 	import citationExtension from '$lib/utils/marked/citation-extension';
 
+	const options = {
+		throwOnError: false,
+		breaks: true
+	};
+
+	marked.use(markedKatexExtension(options));
+	marked.use(markedExtension(options));
+	marked.use(citationExtension(options));
+	marked.use(footnoteExtension(options));
+	marked.use(disableSingleTilde);
+	marked.use({
+		extensions: [mentionExtension({ triggerChar: '@' }), mentionExtension({ triggerChar: '#' })]
+	});
+</script>
+
+<script>
+	import { replaceTokens, processResponseContent } from '$lib/utils';
+	import { user } from '$lib/stores';
+
+	import MarkdownTokens from './Markdown/MarkdownTokens.svelte';
+
 	export let id = '';
 	export let content;
 	export let done = true;
@@ -35,27 +52,11 @@
 
 	let tokens = [];
 
-	const options = {
-		throwOnError: false,
-		breaks: true
-	};
-
-	marked.use(markedKatexExtension(options));
-	marked.use(markedExtension(options));
-	marked.use(citationExtension(options));
-	marked.use(footnoteExtension(options));
-	marked.use(disableSingleTilde);
-	marked.use({
-		extensions: [mentionExtension({ triggerChar: '@' }), mentionExtension({ triggerChar: '#' })]
-	});
-
-	$: (async () => {
-		if (content) {
-			tokens = marked.lexer(
-				replaceTokens(processResponseContent(content), model?.name, $user?.name)
-			);
-		}
-	})();
+	$: if (content) {
+		tokens = marked.lexer(
+			replaceTokens(processResponseContent(content), model?.name, $user?.name)
+		);
+	}
 </script>
 
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