make user sheets popovers on desktop

lib/helpers/extensions/show_adaptive_popover.dart

@@ -0,0 +1,35 @@
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/global_rect.dart";
+import "package:nexus/helpers/extensions/is_desktop.dart";
+import "package:nexus/helpers/extensions/show_adaptive_sheet.dart";
+import "package:nexus/widgets/popover_layout.dart";
+
+extension ShowAdaptivePopover on BuildContext {
+  /// Shows the widget from [builder] in a popover next to the widget this
+  /// context belongs to on desktop, or a bottom sheet on mobile. Use the
+  /// context passed to [builder] to close it.
+  Future<void> showAdaptivePopover(WidgetBuilder builder) {
+    if (!isDesktop) return showAdaptiveSheet(builder);
+
+    final anchor = globalRect;
+
+    return showDialog(
+      context: this,
+      barrierColor: Colors.transparent,
+      useSafeArea: false,
+      builder: (context) => CustomSingleChildLayout(
+        delegate: PopoverLayout(anchor),
+        child: ConstrainedBox(
+          constraints: const .new(maxWidth: 420, maxHeight: 600),
+          child: Material(
+            elevation: 8,
+            clipBehavior: .antiAlias,
+            color: Theme.of(context).colorScheme.surfaceContainer,
+            borderRadius: .circular(16),
+            child: Builder(builder: builder),
+          ),
+        ),
+      ),
+    );
+  }
+}