chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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

halkyon_learn_flutter/test/widget_test.dart

@@ -1,18 +0,0 @@
-import 'package:flutter_test/flutter_test.dart';
-import 'package:halkyon_learn_flutter/src/feature/home/widget/home_screen.dart';
-
-import 'support/pump_app.dart';
-import 'support/test_dependencies.dart';
-
-void main() {
-  testWidgets('signs in and opens at home, the dashboard, beside the '
-      'library', (tester) async {
-    await pumpApp(tester, testDependencies());
-    expect(find.byType(HomeScreen), findsOneWidget);
-    expect(find.text('Set a study goal'), findsOneWidget);
-
-    await openTab(tester, 'Library');
-
-    expect(find.text('No studysets yet'), findsOneWidget);
-  });
-}