Use surface3 for user message bubble

Drops the accentSubtle theme token in favor of an existing surface layer.
surface3 gives a clear step up from surface0 (chat bg) without the tinted
look the accent fill produced.

packages/app/src/components/message.tsx

@@ -346,7 +346,7 @@ const userMessageStylesheet = StyleSheet.create((theme) => ({
     marginBottom: theme.spacing[4],
   },
   bubble: {
-    backgroundColor: theme.colors.accentSubtle,
+    backgroundColor: theme.colors.surface3,
     borderRadius: theme.borderRadius["2xl"],
     borderTopRightRadius: theme.borderRadius.sm,
     paddingHorizontal: theme.spacing[4],