feat(library): who made a studyset, and whether it is public

Each card's foot now says who made the studyset: "By you", or for a copy
of a published studyset its author's handle. Beside the star, a lock or a
globe says whether it is out on Explore, and opens the Publish sheet.

The instance says which of the person's studysets are published and not
withdrawn (`PublishEndpoint.published`, additive: older apps don't call
it), and the app remembers that in the person's folder, so the library
shows it offline too; publishing, withdrawing and looking one up keep it
current.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_client/lib/src/protocol/client.dart

@@ -634,6 +634,15 @@ class EndpointPublish extends _isc.EndpointRef {
         {'studysetId': studysetId},
       );
 
+  /// Your studysets out on Explore: published and not withdrawn, which the
+  /// library marks as public.
+  _ida.Future<List<_isc.UuidValue>> published() =>
+      caller.callServerEndpoint<List<_isc.UuidValue>>(
+        'publish',
+        'published',
+        {},
+      );
+
   /// Publishes your studyset [studysetId] with everything inside it as its
   /// next release, [notes] saying what changed; at the first, under the
   /// name [slug] in its location, which stays its name from then on.