feat: reduce AI chat font size from sm to 2xs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/components/copilot/chat/AIChatMessage.svelte

@@ -71,7 +71,7 @@
 	{:else}
 		<div
 			class={twMerge(
-				'text-sm py-1 mx-2',
+				'text-2xs py-1 mx-2',
 				message.role === 'user' &&
 					'px-2 border border-gray-300 dark:border-gray-600 bg-gray-50 dark:bg-gray-900 rounded-lg relative group',
 				(message.role === 'assistant' || message.role === 'tool') && 'px-[1px]',

frontend/src/lib/components/copilot/chat/AssistantMessage.svelte

@@ -13,7 +13,7 @@
 </script>
 
 <div
-	class="prose prose-sm dark:prose-invert w-full max-w-full leading-snug space-y-2 prose-ul:!pl-6"
+	class="prose prose-xs dark:prose-invert w-full max-w-full leading-snug space-y-2 prose-ul:!pl-6 !text-2xs"
 >
 	<Markdown
 		md={message.content}