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/test/widget/adaptive_sheet_test.dart

@@ -63,6 +63,36 @@ void main() {
     expect(find.byType(BottomSheet), findsNothing);
   });
 
+  testWidgets('sheetShowsAsDialog says which a sheet of its own will be: '
+      'a dialog from expanded windows on, and on the desktop at every width', (
+    tester,
+  ) async {
+    Future<bool> at(double width) async {
+      tester.view.physicalSize = Size(width, 900);
+      tester.view.devicePixelRatio = 1;
+      late bool shows;
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (context) {
+              shows = sheetShowsAsDialog(context);
+              return const SizedBox();
+            },
+          ),
+        ),
+      );
+      return shows;
+    }
+
+    addTearDown(tester.view.reset);
+    expect(await at(400), isFalse);
+    expect(await at(840), isTrue);
+    debugDefaultTargetPlatformOverride = TargetPlatform.linux;
+    addTearDown(() => debugDefaultTargetPlatformOverride = null);
+    expect(await at(400), isTrue);
+    debugDefaultTargetPlatformOverride = null;
+  });
+
   for (final platform in [
     TargetPlatform.linux,
     TargetPlatform.macOS,