show bottom sheets / dialogs / menus based on screen size

Fixes #76

lib/helpers/extensions/show_adaptive_sheet.dart

@@ -0,0 +1,24 @@
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/is_desktop.dart";
+
+extension ShowAdaptiveSheet on BuildContext {
+  /// Shows the widget from [builder] in a dialog on desktop, or a bottom sheet
+  /// on mobile. Use the context passed to [builder] to close it.
+  Future<void> showAdaptiveSheet(WidgetBuilder builder) => isDesktop
+      ? showDialog(
+          context: this,
+          builder: (context) => Dialog(
+            clipBehavior: .antiAlias,
+            child: ConstrainedBox(
+              constraints: const .new(maxWidth: 450, maxHeight: 600),
+              child: Builder(builder: builder),
+            ),
+          ),
+        )
+      : showModalBottomSheet(
+          context: this,
+          isScrollControlled: true,
+          constraints: .new(maxHeight: MediaQuery.sizeOf(this).height - 80),
+          builder: builder,
+        );
+}