Distinguish user message bubbles with accentSubtle fill

User bubbles previously used surface2, which blended into the surrounding
chat surface. Add an `accentSubtle` theme token (accent darkened, slightly
desaturated, low alpha) and use it for the bubble fill so user messages
read as distinct without shouting.

Fixes #986

packages/app/src/components/message.tsx

@@ -346,7 +346,7 @@ const userMessageStylesheet = StyleSheet.create((theme) => ({
     marginBottom: theme.spacing[4],
   },
   bubble: {
-    backgroundColor: theme.colors.surface2,
+    backgroundColor: theme.colors.accentSubtle,
     borderRadius: theme.borderRadius["2xl"],
     borderTopRightRadius: theme.borderRadius.sm,
     paddingHorizontal: theme.spacing[4],