fix: Remove extra spacing before image preview in group chat

Changed message content container from <p> to <div> to fix invalid HTML
when image previews are embedded. The <p> tag cannot contain block-level
elements like <div>, causing browsers to auto-close the paragraph and
create unwanted spacing between text and image preview.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/static/js/app.js

@@ -676,7 +676,7 @@ function createMessageElement(msg) {
             <span class="message-sender">${escapeHtml(msg.sender)}</span>
             <span class="message-time">${time}</span>
         </div>
-        <p class="message-content">${processMessageContent(msg.content)}</p>
+        <div class="message-content">${processMessageContent(msg.content)}</div>
         ${metaInfo ? `<div class="message-meta">${metaInfo}</div>` : ''}
         ${!msg.is_own ? `
             <div class="mt-1 d-flex gap-1">