show badges totaled in more areas

lib/widgets/count_badge.dart

@@ -0,0 +1,16 @@
+import "package:material_ui/material_ui.dart";
+
+/// Shows [count] in a badge on [child]: nothing if null, a dot if 0, and the
+/// number otherwise.
+class const CountBadge(
+  final int? count, {
+  required final Widget child,
+  super.key,
+}) extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) => switch (count) {
+    null => child,
+    0 => Badge(child: child),
+    final count => Badge.count(count: count, child: child),
+  };
+}