feat(train): signing in to Jellyfin and marking playlists (ADR 0030, D47)

Music's decisions as D47, and its first part: the music screen from the
account sheet, on Android, signs in to the person's Jellyfin with Quick
Connect, else a password, and lists their playlists of music to mark
for running. The token is in the keyring, the server and the marks in
the person's music.json, nothing on Train's server.

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

apps/train/halkyon_train_flutter/test/support/fake_jellyfin.dart

@@ -0,0 +1,99 @@
+import 'dart:convert';
+
+import 'package:halkyon_train_flutter/src/feature/music/data/music_store.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+
+/// A Jellyfin server for tests (ADR 0030, D47), at
+/// `https://music.example.org`: Ada signs in with the password `pw` or with
+/// Quick Connect once [approve]d; her token `token-<n>` holds until
+/// [revoke]d.
+class FakeJellyfin({
+  /// Whether it signs in with Quick Connect.
+  final bool quickConnect = true,
+}) {
+  final requests = <http.Request>[];
+  var _approved = false;
+  var _tokens = 0;
+  final _valid = <String>{};
+  List<Map<String, Object?>> playlists = [
+    {'Id': 'p1', 'Name': 'Tempo', 'MediaType': 'Audio', 'ChildCount': 42},
+    {'Id': 'p2', 'Name': 'Easy', 'MediaType': 'Audio', 'ChildCount': 1},
+  ];
+
+  void approve() => _approved = true;
+
+  void revoke() => _valid.clear();
+
+  /// Tokens given and still valid.
+  Set<String> get valid => {..._valid};
+
+  late final http.Client client = MockClient((request) async {
+    requests.add(request);
+    final token = RegExp(
+      'Token="([^"]+)"',
+    ).firstMatch(request.headers['authorization'] ?? '')?.group(1);
+    http.Response json(Object? body, [int status = 200]) =>
+        http.Response(jsonEncode(body), status);
+    String signIn() {
+      final token = 'token-${++_tokens}';
+      _valid.add(token);
+      return token;
+    }
+
+    switch ('${request.method} ${request.url.path}') {
+      case 'GET /System/Info/Public':
+        return json({
+          'ServerName': 'Home',
+          'Id': 'server-1',
+          'Version': '10.10.7',
+        });
+      case 'GET /QuickConnect/Enabled':
+        return json(quickConnect);
+      case 'POST /QuickConnect/Initiate':
+        return quickConnect
+            ? json({'Code': '123456', 'Secret': 'secret-1'})
+            : json(null, 401);
+      case 'GET /QuickConnect/Connect':
+        return json({'Authenticated': _approved});
+      case 'POST /Users/AuthenticateWithQuickConnect':
+        return _approved
+            ? json({
+                'AccessToken': signIn(),
+                'User': {'Id': 'user-1', 'Name': 'Ada'},
+              })
+            : json(null, 401);
+      case 'POST /Users/AuthenticateByName':
+        final body = jsonDecode(request.body) as Map<String, Object?>;
+        return body['Username'] == 'Ada' && body['Pw'] == 'pw'
+            ? json({
+                'AccessToken': signIn(),
+                'User': {'Id': 'user-1', 'Name': 'Ada'},
+              })
+            : json(null, 401);
+      case 'GET /Items':
+        return _valid.contains(token)
+            ? json({'Items': playlists})
+            : json(null, 401);
+      case 'POST /Sessions/Logout':
+        _valid.remove(token);
+        return http.Response('', 204);
+    }
+    return http.Response('', 404);
+  });
+}
+
+/// A [JellyfinTokenStore] in memory, with others' tokens in [others].
+class MemoryJellyfinTokenStore() implements JellyfinTokenStore {
+  String? token;
+  final others = <String>{'someone-else'};
+
+  @override
+  Future<String?> read() async => token;
+
+  @override
+  Future<void> keep(String? token) async => this.token = token;
+
+  @override
+  Future<void> forgetOthers() async => others.clear();
+}