feat: back is greyed while an import runs, so nothing cuts it off

A sheet can be busy (SheetScaffold.busy): its close button is off and
greyed, and back does nothing until it ends. The Anki import's sheet is
busy while it imports; before, its close button looked as always and
did nothing.

A studyset's screen, while it imports a PDF or a path, greys its back
button and the path's steps and ignores back: leaving would close the
lists' blocs and cut the import off. Importing a path now says so in its
state, as a PDF's did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/core/widget/adaptive_sheet_test.dart

@@ -12,6 +12,7 @@ void main() {
     required double width,
     Widget body = const Text('Body'),
     double statusBar = 0,
+    bool busy = false,
   }) async {
     tester.view.physicalSize = Size(width, 900);
     tester.view.devicePixelRatio = 1;
@@ -27,8 +28,10 @@ void main() {
             builder: (context) => TextButton(
               onPressed: () => showAdaptiveSheet<void>(
                 context,
+                swipeable: !busy,
                 builder: (context) => SheetScaffold(
                   title: 'New studyset',
+                  busy: busy,
                   body: body,
                   footer: FilledButton(
                     onPressed: () {},
@@ -105,6 +108,27 @@ void main() {
     expect(find.text('New studyset'), findsNothing);
   });
 
+  testWidgets('while busy, its close button is off and back keeps it open', (
+    tester,
+  ) async {
+    await open(tester, width: 400, busy: true);
+
+    expect(
+      tester
+          .widget<IconButton>(
+            find.ancestor(
+              of: find.byTooltip('Close'),
+              matching: find.byType(IconButton),
+            ),
+          )
+          .onPressed,
+      isNull,
+    );
+    await tester.binding.handlePopRoute();
+    await tester.pumpAndSettle();
+    expect(find.text('New studyset'), findsOneWidget);
+  });
+
   group('a body longer than the sheet', () {
     final tall = Column(
       children: [for (var i = 0; i < 40; i++) const Text('Line')],