feat(profile): a handle in a studyset opens its person's profile

The summary's first block now reads as a studyset's card: its secondary
title, its title, then by @handle; the blocks' second heading is
Material. A licence holder written as a handle of the instance
(@raisondetredev) reads © raisondetredev, the © standing for the @,
and is a link, apart from a name from elsewhere such as Tae Kim's; it
and the author open their public profile in a sheet: avatar, name,
address, bio and links, as the instance's profiles.byHandle gives them.

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

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -1739,6 +1739,17 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get publicProfileTitle => 'Öffentliches Profil';
 
+  @override
+  String get profileSheetTitle => 'Profil';
+
+  @override
+  String get profileSheetNotFound =>
+      'Niemand auf dieser Instanz hat diesen Handle.';
+
+  @override
+  String get profileSheetUnreachable =>
+      'Das Profil konnte nicht geladen werden. Prüfe deine Verbindung.';
+
   @override
   String get publicProfileIntro =>
       'Wie andere dich sehen, sobald du veröffentlichst: das Handle, unter dem sie dich finden, dein Name, ein paar Worte und Links. Es speichert sich beim Schreiben, sobald du dein Handle gewählt hast.';
@@ -6045,7 +6056,7 @@ class AppLocalizationsDe extends AppLocalizations {
   }
 
   @override
-  String get exploreHolds => 'Inhalt';
+  String get exploreMaterial => 'Material';
 
   @override
   String exploreLatestRelease(int version, String date) {