feat(learn): share a studyset with people from its Share sheet

Share opens one sheet (Halkyon's ADR 0044, D29): the people first, then
the community. A studyset of the person's own offers Share with people,
which says what it does, moves it into a space of its own with its
progress told and Abort, and opens it there (StudysetSharingCubit). A
shared one lists its members (SpaceMembersCubit): the Owner invites by
handle as Editor or Viewer, changes roles, hands on and removes, and
deletes it for everyone; anyone leaves, the Owner naming who takes it
on; each refusal is said. A member's row opens their safety code
(twelve groups of five digits) to mark compared, and changed keys are
flagged, trusted only after comparing (D32). The community row leads to
the privacy dialog or publishing as Share did, and is off for a shared
studyset (D27).

A studyset knows its space and whether the person only views it
(Studyset.sharedIn, viewOnly, from the sync engine's grants). Its mark
is Lucide's users icon, "Shared · private" (D31). A Viewer's studyset
screen, its options, its topics and the move sheet offer nothing that
writes, and say they can view and study it (D30); the engine refuses
their writes on the device in any case. A shared top isn't moved,
archived or deleted from its options, and nothing moves in or out of
its space.

Whats-New: Share a studyset with people: invite them by their handle as editors or viewers, and compare safety codes to know nobody stands between you.
Whats-New-De: Teile ein Lernset mit Personen: Lade sie über ihr Handle als Bearbeiter:innen oder Betrachter:innen ein und vergleicht Sicherheitscodes, damit niemand zwischen euch steht.

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/data/studyset_repository_test.dart

@@ -370,4 +370,22 @@ void main() {
 
     expect(titles(await repository.watchAll().first), ['Readable']);
   });
+
+  test("a studyset in a shared space says so, and whether it's the "
+      "person's to change there (ADR 0044, D30, D31)", () async {
+    final own = await repository.create(StudysetContent.titled('Own'));
+    final shared = await repository.create(StudysetContent.titled('Shared'));
+    final viewed = await repository.create(StudysetContent.titled('Viewed'));
+    gateway.shared
+      ..[shared] = (space: 'space-1', viewOnly: false)
+      ..[viewed] = (space: 'space-2', viewOnly: true);
+
+    final all = {for (final s in await repository.withDeleted()) s.id: s};
+    expect(all[own]!.isShared, isFalse);
+    expect(all[own]!.viewOnly, isFalse);
+    expect(all[shared]!.sharedIn, 'space-1');
+    expect(all[shared]!.viewOnly, isFalse);
+    expect(all[viewed]!.sharedIn, 'space-2');
+    expect(all[viewed]!.viewOnly, isTrue);
+  });
 }