show bottom sheets / dialogs / menus based on screen size

Fixes #76

lib/widgets/adaptive_menu_button.dart

@@ -0,0 +1,21 @@
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/show_adaptive_menu.dart";
+
+/// Like [PopupMenuButton], but shows a bottom sheet on mobile.
+class const AdaptiveMenuButton({
+  required final Widget icon,
+  required final List<PopupMenuEntry> Function(BuildContext context)
+  itemBuilder,
+  final String? tooltip,
+  final bool enabled = true,
+  super.key,
+}) extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) => IconButton(
+    icon: icon,
+    tooltip: tooltip,
+    onPressed: enabled
+        ? () => context.showAdaptiveMenu(itemBuilder(context))
+        : null,
+  );
+}