webui: add lazy markdown loading for improved performance

Add LazyMarkdownContent component that defers markdown rendering until
messages scroll into view. This significantly improves load times for
conversations with many messages (500+ messages would previously timeout).

Changes:
- New LazyMarkdownContent.svelte using IntersectionObserver
- ChatMessageAssistant uses lazy loading for completed messages,
  eager rendering for streaming messages
- ChatMessageUser and ChatMessageSystem use lazy loading
- Short messages (<200 chars) render immediately for responsiveness

The component shows a lightweight text placeholder until the message
enters the viewport (with 200px margin for preloading), then renders
full markdown with syntax highlighting, KaTeX, etc.

tools/server/webui/src/lib/components/app/chat/ChatMessages/ChatMessageAssistant.svelte

@@ -5,6 +5,7 @@
 		ChatMessageStatistics,
 		ChatMessageThinkingBlock,
 		CopyToClipboardIcon,
+		LazyMarkdownContent,
 		MarkdownContent,
 		ModelsSelector
 	} from '$lib/components/app';
@@ -240,7 +241,11 @@
 	{:else if message.role === 'assistant'}
 		{#if config().disableReasoningFormat}
 			<pre class="raw-output">{messageContent || ''}</pre>
+		{:else if message.timestamp && !isLoading()}
+			<!-- Completed message: use lazy loading for performance with many messages -->
+			<LazyMarkdownContent content={messageContent || ''} />
 		{:else}
+			<!-- Streaming message: render immediately -->
 			<MarkdownContent content={messageContent || ''} />
 		{/if}
 	{:else}

tools/server/webui/src/lib/components/app/chat/ChatMessages/ChatMessageSystem.svelte

@@ -2,7 +2,7 @@
 	import { Check, X } from '@lucide/svelte';
 	import { Card } from '$lib/components/ui/card';
 	import { Button } from '$lib/components/ui/button';
-	import { MarkdownContent } from '$lib/components/app';
+	import { LazyMarkdownContent } from '$lib/components/app';
 	import { INPUT_CLASSES } from '$lib/constants/input-classes';
 	import { config } from '$lib/stores/settings.svelte';
 	import ChatMessageActions from './ChatMessageActions.svelte';
@@ -145,7 +145,7 @@
 						>
 							{#if currentConfig.renderUserContentAsMarkdown}
 								<div bind:this={messageElement} class="text-md {isExpanded ? 'cursor-text' : ''}">
-									<MarkdownContent class="markdown-system-content" content={message.content} />
+									<LazyMarkdownContent class="markdown-system-content" content={message.content} />
 								</div>
 							{:else}
 								<span

tools/server/webui/src/lib/components/app/chat/ChatMessages/ChatMessageUser.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { Card } from '$lib/components/ui/card';
-	import { ChatAttachmentsList, MarkdownContent } from '$lib/components/app';
+	import { ChatAttachmentsList, LazyMarkdownContent, MarkdownContent } from '$lib/components/app';
 	import { config } from '$lib/stores/settings.svelte';
 	import ChatMessageActions from './ChatMessageActions.svelte';
 	import ChatMessageEditForm from './ChatMessageEditForm.svelte';
@@ -128,7 +128,7 @@
 			>
 				{#if currentConfig.renderUserContentAsMarkdown}
 					<div bind:this={messageElement} class="text-md">
-						<MarkdownContent
+						<LazyMarkdownContent
 							class="markdown-user-content text-primary-foreground"
 							content={message.content}
 						/>

tools/server/webui/src/lib/components/app/index.ts

@@ -62,6 +62,7 @@ export { default as BadgeModality } from './misc/BadgeModality.svelte';
 export { default as ConversationSelection } from './misc/ConversationSelection.svelte';
 export { default as CopyToClipboardIcon } from './misc/CopyToClipboardIcon.svelte';
 export { default as KeyboardShortcutInfo } from './misc/KeyboardShortcutInfo.svelte';
+export { default as LazyMarkdownContent } from './misc/LazyMarkdownContent.svelte';
 export { default as MarkdownContent } from './misc/MarkdownContent.svelte';
 export { default as RemoveButton } from './misc/RemoveButton.svelte';
 export { default as SearchInput } from './misc/SearchInput.svelte';

tools/server/webui/src/lib/components/app/misc/LazyMarkdownContent.svelte

@@ -0,0 +1,98 @@
+<script lang="ts">
+	import { onMount, onDestroy } from 'svelte';
+	import MarkdownContent from './MarkdownContent.svelte';
+
+	interface Props {
+		content: string;
+		class?: string;
+		/** If true, render immediately without waiting for intersection */
+		eager?: boolean;
+		/** Placeholder height estimate for layout stability */
+		placeholderHeight?: string;
+	}
+
+	let { content, class: className = '', eager = false, placeholderHeight = 'auto' }: Props = $props();
+
+	let containerRef = $state<HTMLDivElement>();
+	let isVisible = $state(false);
+	let hasBeenVisible = $state(false);
+	let observer: IntersectionObserver | null = null;
+	
+	// Initialize based on eager prop
+	$effect(() => {
+		if (eager) {
+			isVisible = true;
+			hasBeenVisible = true;
+		}
+	});
+
+	onMount(() => {
+		if (eager) {
+			hasBeenVisible = true;
+			isVisible = true;
+			return;
+		}
+		
+		if (!containerRef) {
+			return;
+		}
+
+		observer = new IntersectionObserver(
+			(entries) => {
+				for (const entry of entries) {
+					if (entry.isIntersecting) {
+						isVisible = true;
+						hasBeenVisible = true;
+						// Once visible, no need to observe anymore
+						observer?.disconnect();
+					}
+				}
+			},
+			{
+				// Start loading slightly before element comes into view
+				rootMargin: '200px 0px',
+				threshold: 0
+			}
+		);
+
+		observer.observe(containerRef);
+	});
+
+	onDestroy(() => {
+		observer?.disconnect();
+	});
+
+	// If content is very short, render eagerly (no point lazy loading)
+	$effect(() => {
+		if (content.length < 200 && !hasBeenVisible) {
+			hasBeenVisible = true;
+			isVisible = true;
+		}
+	});
+</script>
+
+<div 
+	bind:this={containerRef} 
+	class={className}
+	style:min-height={!hasBeenVisible ? placeholderHeight : 'auto'}
+>
+	{#if hasBeenVisible}
+		<MarkdownContent {content} class={className} />
+	{:else}
+		<!-- Lightweight placeholder: show raw text truncated -->
+		<div class="placeholder-content">
+			{content.slice(0, 500)}{content.length > 500 ? '...' : ''}
+		</div>
+	{/if}
+</div>
+
+<style>
+	.placeholder-content {
+		white-space: pre-wrap;
+		word-break: break-word;
+		color: var(--muted-foreground);
+		font-size: 0.875rem;
+		line-height: 1.5;
+		opacity: 0.7;
+	}
+</style>