feat(app): align interface typography to base size

Treat the appearance setting as the rendered base size so the type ramp scales predictably. Native starts one pixel larger while web and desktop preserve the authored 14px base, with legacy values migrated on load.

packages/app/src/agent-stream/chat-outline/rail.web.tsx

@@ -249,7 +249,7 @@ const styles = StyleSheet.create((theme) => ({
     ...theme.shadow.md,
   },
   previewText: {
-    fontSize: theme.fontSize.xs,
+    fontSize: theme.fontSize.sm,
     color: theme.colors.foreground,
   },
 }));