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/test/support/fake_profile_gateway.dart

@@ -12,7 +12,18 @@ class FakeProfileGateway({
 
   /// Handles others have.
   final Set<String> taken = const {},
+
+  /// Others' profiles, by their handles.
+  final Map<String, PublicProfile> others = const {},
 }) implements ProfileGateway {
+  @override
+  Future<PublicProfile?> byHandle(String handle) async {
+    if (fail case final error?) {
+      throw error;
+    }
+    return others[handle.toLowerCase()];
+  }
+
   /// When set, every request fails with it.
   Exception? fail;