feat(ai-chat): shrink chat message font size

Reduce the default font size of chat messages to create a smaller, denser view. This adjustment cascades into user bubbles, assistant markdown, and tool cards while preserving the size of explicit Mantine labels. Improves readability and allows more content to fit within the chat window.

apps/client/src/features/ai-chat/components/ai-chat.module.css

@@ -8,6 +8,9 @@
 .messages {
     flex: 1 1 auto;
     min-height: 0;
+    /* Smaller, denser chat text (cascades into user bubble + assistant markdown
+       + tool cards; the explicit-size Mantine labels keep their own size). */
+    font-size: var(--mantine-font-size-sm);
 }
 
 .messageRow {