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/feature/explore/explore_screen_test.dart

@@ -9,6 +9,7 @@ import 'package:halkyon_learn_flutter/src/feature/auth/model/auth_status.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_payload.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/model/card_content.dart';
 import 'package:halkyon_learn_flutter/src/feature/explore/model/explore_studyset.dart';
+import 'package:halkyon_learn_flutter/src/feature/profile/model/public_profile.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/data/studyset_payload.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/data/studyset_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
@@ -17,6 +18,7 @@ import 'package:material_ui/material_ui.dart';
 
 import '../../support/fake_auth_gateway.dart';
 import '../../support/fake_explore_gateway.dart';
+import '../../support/fake_profile_gateway.dart';
 import '../../support/fake_studyset_gateway.dart';
 import '../../support/pump_app.dart';
 import '../../support/test_dependencies.dart';
@@ -91,6 +93,7 @@ void main() {
     WidgetTester tester,
     FakeExploreGateway explore, {
     bool identified = true,
+    FakeProfileGateway? profiles,
   }) async {
     tester.view.physicalSize = const Size(900, 1400);
     tester.view.devicePixelRatio = 1;
@@ -106,6 +109,7 @@ void main() {
         ),
         studysetGateway: studysets,
         exploreGateway: explore,
+        profileGateway: profiles,
       ),
       location: RoutePaths.explore,
     );
@@ -242,6 +246,51 @@ void main() {
     );
   });
 
+  testWidgets('a licence holder of the instance is named by their handle, '
+      "without the @, opening their profile; one from elsewhere isn't", (
+    tester,
+  ) async {
+    final japanese = published('Japanese');
+    await showExplore(
+      tester,
+      FakeExploreGateway(
+        published: [
+          ExploreStudyset(
+            publicId: japanese.publicId,
+            location: japanese.location,
+            handle: 'alice',
+            version: 3,
+            releasedAt: japanese.releasedAt,
+            title: 'Japanese',
+            description: japanese.description,
+            license: 'CC-BY-SA-4.0',
+            licenseHolder: '@alice',
+          ),
+        ],
+      ),
+      profiles: FakeProfileGateway(
+        others: {
+          'alice': const PublicProfile(
+            handle: 'alice',
+            address: 'alice@halkyon.eu',
+            displayName: 'Alice',
+            bio: 'Learning Japanese.',
+          ),
+        },
+      ),
+    );
+    await tester.tap(find.text('Japanese'));
+    await tester.pumpAndSettle();
+
+    expect(find.textContaining('@alice', findRichText: true), findsWidgets);
+    expect(find.text('alice'), findsOneWidget);
+    await tester.tap(find.text('alice'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Learning Japanese.'), findsOneWidget);
+    expect(find.text('@alice@halkyon.eu'), findsOneWidget);
+  });
+
   testWidgets('a summary shows all of it, and adds it to the library as '
       'a copy, opened from there', (tester) async {
     await showExplore(

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;