make user sheets popovers on desktop

lib/helpers/extensions/global_rect.dart

@@ -0,0 +1,15 @@
+import "package:material_ui/material_ui.dart";
+
+extension GlobalRect on BuildContext {
+  /// Where the widget this context belongs to is on screen, e.g. to open a
+  /// menu or popover next to it.
+  Rect get globalRect {
+    final overlay = Overlay.of(this).context.findRenderObject() as RenderBox;
+
+    return switch (findRenderObject()) {
+      final RenderBox box =>
+        box.localToGlobal(.zero, ancestor: overlay) & box.size,
+      _ => Rect.zero,
+    };
+  }
+}