feat(learn): spelling and grammar checked as people write (ADR 0024)

Where the instance's LanguageTool checks the language, the editors
underline what it finds after a pause, a paragraph at a time, a tap
offering its suggestions (GrammarHost, GrammarCheckController,
GrammarOverlay; nothing written into the document). A card's front is
checked in the studyset's front language, its back and answers in the
back's, everything else in the language recognised.

In a private studyset nothing is sent until the person agrees for it,
asked once in a banner above the editor and kept in their settings
(an agreement under privateAgreements, a refusal in grammarDeclined,
settings schema 14), taken back in the studyset's privacy dialog.
Settings > Writing turns it off on the device.

The field's Check asks LanguageTool first where it checks the
studyset's language, in a sheet offering to ask the assistant why;
elsewhere it asks the assistant as before (amends ADR 0021, D54).

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

apps/learn/halkyon_learn_flutter/test/feature/settings/widget/settings_screen_test.dart

@@ -261,6 +261,53 @@ void main() {
     expect(store.writes.last.pomodoroWhileStudying, isFalse);
   });
 
+  testWidgets('the Writing card turns off checking what is written (ADR '
+      '0024), and says where the instance checks nothing yet', (tester) async {
+    Future<void> show(
+      List<String>? languages,
+      MemoryDeviceSettingsStore store,
+    ) => pumpApp(
+      tester,
+      testDependencies(
+        deviceSettingsStore: store,
+        grammarLanguages: languages,
+      ),
+      location: RoutePaths.settings,
+    );
+    const title = 'Check grammar while writing';
+    const none =
+        'This instance has no spelling and grammar check set up yet, so '
+        'nothing is underlined. Left on, checking starts once it has one.';
+
+    final store = MemoryDeviceSettingsStore();
+    await show(['en', 'de'], store);
+    final toggle = find.widgetWithText(SwitchListTile, title);
+    await tester.scrollUntilVisible(
+      toggle,
+      200,
+      scrollable: find.byType(Scrollable).first,
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('Writing'), findsOneWidget);
+    expect(tester.widget<SwitchListTile>(toggle).value, isTrue);
+    expect(find.text(none), findsNothing);
+
+    await tester.tap(
+      find.descendant(of: toggle, matching: find.byType(Switch)),
+    );
+    await tester.pumpAndSettle();
+    expect(store.writes.last.checkGrammar, isFalse);
+
+    await tester.pumpWidget(const SizedBox());
+    await show(null, MemoryDeviceSettingsStore());
+    await tester.scrollUntilVisible(
+      find.text(none),
+      200,
+      scrollable: find.byType(Scrollable).first,
+    );
+    expect(find.text(none), findsOneWidget);
+  });
+
   testWidgets('starts with the appearance the device kept', (tester) async {
     tester.platformDispatcher.platformBrightnessTestValue = Brightness.dark;
     addTearDown(tester.platformDispatcher.clearPlatformBrightnessTestValue);