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/card/widget/card_composer_test.dart

@@ -18,6 +18,7 @@ import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:material_ui/material_ui.dart' hide Card;
 
 import '../../../support/fake_card_gateway.dart';
+import '../../../support/fake_grammar_gateway.dart';
 import '../../../support/fake_studyset_gateway.dart';
 import '../../../support/loaded_maps.dart';
 import '../../../support/pump_app.dart';
@@ -90,6 +91,9 @@ void main() {
     GeoMapRepository? maps,
     Size size = const Size(1400, 1600),
     String? frontLanguage,
+    String? backLanguage,
+    FakeGrammarGateway? grammar,
+    List<String>? grammarLanguages,
   }) async {
     tester.view.physicalSize = size;
     tester.view.devicePixelRatio = 1;
@@ -102,6 +106,7 @@ void main() {
             title: 'Japanese',
             description: RichContent.empty(),
             frontLanguage: frontLanguage,
+            backLanguage: backLanguage,
           ),
         ),
       );
@@ -111,6 +116,8 @@ void main() {
         studysetGateway: studysets,
         cardGateway: gateway,
         maps: maps,
+        grammarGateway: grammar,
+        grammarLanguages: grammarLanguages,
       ),
       location: RoutePaths.studyset('japanese'),
     );
@@ -162,6 +169,30 @@ void main() {
     expect(gateway.written, isEmpty);
   });
 
+  testWidgets("a card's front is checked as it is written in the "
+      "studyset's front language, its back in the back's (ADR 0024)", (
+    tester,
+  ) async {
+    final grammar = FakeGrammarGateway();
+    await cards.create('japanese', card('猫', 'cat'));
+    await showStudyset(
+      tester,
+      frontLanguage: 'ja',
+      backLanguage: 'en-GB',
+      grammar: grammar,
+      grammarLanguages: ['ja', 'en'],
+    );
+    await open(tester, '猫');
+
+    await tester.tap(inField('猫'));
+    await settle(tester);
+    expect(grammar.checked, [('猫', 'ja')]);
+
+    await tester.tap(inField('cat'));
+    await settle(tester);
+    expect(grammar.checked.last, ('cat', 'en-GB'));
+  });
+
   testWidgets('a card opens in the composer among the others; an edit is '
       'saved on leaving it', (tester) async {
     await cards.create('japanese', card('猫', 'cat'));