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withdrawn (`PublishEndpoint.published`, additive: older apps don't call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_flutter/lib/l10n/app_localizations_de.dart

@@ -219,6 +219,23 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get studysetUnstar => 'Markierung entfernen';
 
+  @override
+  String get studysetByYou => 'Von dir';
+
+  @override
+  String studysetByAuthor(String author) {
+    return 'Von $author';
+  }
+
+  @override
+  String get studysetByOthers => 'Eine Kopie';
+
+  @override
+  String get studysetVisibilityPrivate => 'Privat';
+
+  @override
+  String get studysetVisibilityPublic => 'Öffentlich';
+
   @override
   String get studysetOptions => 'Optionen';