feat(learn): the assistant's conversation a dialog on the desktop and in expanded windows (Todo's ADR 0033, D11)

- halkyon_ui's sheetShowsAsDialog says whether showAdaptiveSheet shows
  a dialog here, for a sheet of its own that can't go through it.
- Learn's conversation, a half-height sheet dragged up, asks it: where
  sheets are dialogs it is a dialog up to 720 wide and as tall as the
  window, with no handle to drag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/lib/src/widget/adaptive_sheet.dart

@@ -28,7 +28,7 @@ Future<T?> showAdaptiveSheet<T>(
   required WidgetBuilder builder,
   bool swipeable = true,
 }) {
-  if (!context.windowSize.isAtLeastExpanded && !_onDesktop) {
+  if (!sheetShowsAsDialog(context)) {
     return showModalBottomSheet<T>(
       context: context,
       useRootNavigator: true,
@@ -63,6 +63,13 @@ Future<T?> showAdaptiveSheet<T>(
   );
 }
 
+/// Whether [showAdaptiveSheet] shows a sheet as a dialog here: from
+/// expanded windows on, and on the desktop at every width. For a sheet of
+/// its own that can't go through it (one dragged to its height, as Learn's
+/// conversation), so that it follows the same rule.
+bool sheetShowsAsDialog(BuildContext context) =>
+    context.windowSize.isAtLeastExpanded || _onDesktop;
+
 /// Whether the app runs on the desktop, where sheets are dialogs at every
 /// width.
 bool get _onDesktop => switch (defaultTargetPlatform) {