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/studyset/widget/studyset_privacy_options_test.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_actions.dart';
+
+/// Privacy in a studyset's options (ADR 0019): chosen at the top level,
+/// and a locked studyset can only be deleted.
+void main() {
+  final l10n = lookupAppLocalizations(const Locale('en'));
+
+  Set<StudysetOption> optionsOf(Studyset studyset) => {
+    for (final entry in studysetOptions(l10n, studyset)) entry.option,
+  };
+
+  test('a readable top-level studyset can be made private, a private one '
+      'non-private', () {
+    final readable = Studyset(
+      id: 's',
+      content: StudysetContent.titled('Diary'),
+    );
+    final private = Studyset(
+      id: 's',
+      content: StudysetContent.titled('Diary'),
+      isPrivate: true,
+    );
+
+    expect(optionsOf(readable), contains(StudysetOption.makePrivate));
+    expect(optionsOf(readable), isNot(contains(StudysetOption.makeReadable)));
+    expect(optionsOf(private), contains(StudysetOption.makeReadable));
+    expect(optionsOf(private), isNot(contains(StudysetOption.makePrivate)));
+  });
+
+  test("a topic's privacy is its tree's: it offers neither", () {
+    final topic = Studyset(
+      id: 't',
+      content: StudysetContent.titled('Kanji'),
+      parentId: 's',
+      isPrivate: true,
+    );
+
+    expect(
+      optionsOf(topic),
+      isNot(
+        anyOf(
+          contains(StudysetOption.makePrivate),
+          contains(StudysetOption.makeReadable),
+        ),
+      ),
+    );
+  });
+
+  test('a locked studyset offers nothing but deleting', () {
+    final locked = Studyset(
+      id: 's',
+      content: StudysetContent.titled(''),
+      isPrivate: true,
+      isLocked: true,
+    );
+
+    expect(optionsOf(locked), {StudysetOption.delete});
+  });
+}