feat(studyset): make a studyset private or not, and what a locked one shows (ADR 0019)

- A top-level studyset's options make it private, with everything inside
  it, or non-private again, after a dialog saying what changes and that
  the instance's backups keep readable copies until they expire; a device
  without the account's keys says how to get them there.
- A locked studyset (its key doesn't open here) is titled Locked, says
  why when opened, and offers only Delete.
- Publishing names the person's tags as the device reads them, and says
  what to do when the studyset is private or a tag has no name.
- The export sheet says an archive is readable by whoever has the file.

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/publish/data/publish_tag_names_test.dart

@@ -0,0 +1,33 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/feature/publish/data/publish_repository.dart';
+
+import '../../../support/fake_publish_gateway.dart';
+import '../../../support/memory_published_store.dart';
+
+/// The instance can't read the person's tags (ADR 0019, D2): publishing
+/// names them as this device reads them.
+void main() {
+  test("publishing sends the tags' names this device reads", () async {
+    final gateway = FakePublishGateway();
+    final publishing = PublishRepository(
+      gateway,
+      MemoryPublishedStore(),
+      tagNames: () async => {'tag-1': 'JLPT-N5'},
+    );
+    addTearDown(publishing.dispose);
+
+    await publishing.publish('studyset-1', slug: 'japanese', notes: '');
+
+    expect(gateway.lastTagNames, {'tag-1': 'JLPT-N5'});
+  });
+
+  test('without names to give, none are sent', () async {
+    final gateway = FakePublishGateway();
+    final publishing = PublishRepository(gateway, MemoryPublishedStore());
+    addTearDown(publishing.dispose);
+
+    await publishing.publish('studyset-1', slug: 'japanese', notes: '');
+
+    expect(gateway.lastTagNames, isNull);
+  });
+}