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_en.dart

@@ -1724,6 +1724,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get publicProfileTitle => 'Public profile';
 
+  @override
+  String get profileSheetTitle => 'Profile';
+
+  @override
+  String get profileSheetNotFound => 'Nobody on this instance has this handle.';
+
+  @override
+  String get profileSheetUnreachable =>
+      'The profile couldn’t be loaded. Check your connection.';
+
   @override
   String get publicProfileIntro =>
       'How others see you once you publish: the handle they find you by, your name, a few words and links. It saves as you write, once you have chosen your handle.';
@@ -5992,7 +6002,7 @@ class AppLocalizationsEn extends AppLocalizations {
   }
 
   @override
-  String get exploreHolds => 'What it holds';
+  String get exploreMaterial => 'Material';
 
   @override
   String exploreLatestRelease(int version, String date) {