ui: fix collapsed user bubble with markdown rendering (#25869)

Edge paragraph margins are now zeroed at the source in
markdown-content.css, but the user bubble and the system message still
carried the -my-4 compensation for them. The uncompensated negative
margins shrank the wrapper 2rem below its content, collapsing
single-line user bubbles into a scrollable sliver and skewing the
system message expand threshold.

tools/ui/src/lib/components/app/chat/ChatMessages/ChatMessage/ChatMessageSystem/ChatMessageSystem.svelte

@@ -157,10 +157,7 @@
 						>
 							{#if currentConfig.renderUserContentAsMarkdown}
 								<div bind:this={messageElement} class={isExpanded ? 'cursor-text' : ''}>
-									<MarkdownContent
-										class="markdown-system-content -my-4"
-										content={message.content}
-									/>
+									<MarkdownContent class="markdown-system-content" content={message.content} />
 								</div>
 							{:else}
 								<span

tools/ui/src/lib/components/app/chat/ChatMessages/ChatMessage/ChatMessageUser/ChatMessageUserBubble.svelte

@@ -65,7 +65,7 @@
 	>
 		{#if renderMarkdown && currentConfig.renderUserContentAsMarkdown}
 			<div bind:this={messageElement}>
-				<MarkdownContent class="markdown-user-content -my-4" {content} />
+				<MarkdownContent class="markdown-user-content" {content} />
 			</div>
 		{:else}
 			<span bind:this={messageElement} class="text-md whitespace-pre-wrap">