halkyon_app: instances, sign-in and sync plumbing for Halkyon's apps

What every Halkyon app's shell needs, taken from Halkyon Learn's and set
on halkyon_base's client: the instances a device keeps an account on,
the first start's tiles and where an instance keeps people's data, the
account switcher; signing in anonymously, with GitHub or an email code,
switching person and the start screen; the gates that build an app's
dependencies per instance and per person; and the sync service with its
status, Settings card and banner. Texts of its own in English and
German take the app's name; what only the app can say, it passes in.
Fakes for apps' tests are in testing.dart.

An instance's basics now name the app it serves (`app`, set by the
host's configuration), and an app accepts only an instance of its own.
SerialQueue and CoalescingRunner move into halkyon_core.

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

packages/halkyon_base_client/lib/src/protocol/client.dart

@@ -197,7 +197,7 @@ class EndpointInstance extends _isc.EndpointRef {
   @override
   String get name => 'halkyon_base.instance';
 
-  /// This instance's basics: GitHub sign-in only with the operator's OAuth
+  /// This instance's basics: the app it serves, GitHub sign-in only with the operator's OAuth
   /// app, email sign-in only where it can mail codes, where its source is,
   /// the oldest app build it works with, and where it keeps people's data
   /// and its backups.

packages/halkyon_base_client/lib/src/protocol/instance/instance_basics.dart

@@ -13,12 +13,13 @@
 import 'package:serverpod_client/serverpod_client.dart' as _isc;
 
 /// What every Halkyon instance tells its apps, to anyone, before anyone signs
-/// in: how people sign in here, where the source of what it runs is, the
+/// in: which app it serves, how people sign in here, where the source of what it runs is, the
 /// oldest app build it works with, and where it keeps people's data. Every
 /// instance is run by someone else, with its own configuration.
 abstract class InstanceBasics
     implements _isc.SerializableModel, _isc.ProtocolSerialization {
   InstanceBasics._({
+    required this.app,
     this.gitHubClientId,
     required this.emailSignIn,
     required this.sourceCodeUrl,
@@ -28,6 +29,7 @@ abstract class InstanceBasics
   });
 
   factory InstanceBasics({
+    required String app,
     String? gitHubClientId,
     required bool emailSignIn,
     required String sourceCodeUrl,
@@ -38,6 +40,7 @@ abstract class InstanceBasics
 
   factory InstanceBasics.fromJson(Map<String, dynamic> jsonSerialization) {
     return InstanceBasics(
+      app: jsonSerialization['app'] as String,
       gitHubClientId: jsonSerialization['gitHubClientId'] as String?,
       emailSignIn: _isc.BoolJsonExtension.fromJson(
         jsonSerialization['emailSignIn'],
@@ -49,6 +52,10 @@ abstract class InstanceBasics
     );
   }
 
+  /// Which app this instance serves (`learn` for Halkyon Learn), as its host
+  /// configures it: an app accepts only an instance of its own.
+  String app;
+
   /// The client ID of this instance's GitHub OAuth app, or null when GitHub
   /// sign-in is off here.
   String? gitHubClientId;
@@ -79,6 +86,7 @@ abstract class InstanceBasics
   /// with some or all fields replaced by the given arguments.
   @_isc.useResult
   InstanceBasics copyWith({
+    String? app,
     String? gitHubClientId,
     bool? emailSignIn,
     String? sourceCodeUrl,
@@ -90,6 +98,7 @@ abstract class InstanceBasics
   Map<String, dynamic> toJson() {
     return {
       '__className__': 'halkyon_base.InstanceBasics',
+      'app': app,
       if (gitHubClientId != null) 'gitHubClientId': gitHubClientId,
       'emailSignIn': emailSignIn,
       'sourceCodeUrl': sourceCodeUrl,
@@ -103,6 +112,7 @@ abstract class InstanceBasics
   Map<String, dynamic> toJsonForProtocol() {
     return {
       '__className__': 'halkyon_base.InstanceBasics',
+      'app': app,
       if (gitHubClientId != null) 'gitHubClientId': gitHubClientId,
       'emailSignIn': emailSignIn,
       'sourceCodeUrl': sourceCodeUrl,
@@ -122,6 +132,7 @@ class _Undefined {}
 
 class _InstanceBasicsImpl extends InstanceBasics {
   _InstanceBasicsImpl({
+    required String app,
     String? gitHubClientId,
     required bool emailSignIn,
     required String sourceCodeUrl,
@@ -129,6 +140,7 @@ class _InstanceBasicsImpl extends InstanceBasics {
     String? dataLocation,
     String? backupLocation,
   }) : super._(
+         app: app,
          gitHubClientId: gitHubClientId,
          emailSignIn: emailSignIn,
          sourceCodeUrl: sourceCodeUrl,
@@ -142,6 +154,7 @@ class _InstanceBasicsImpl extends InstanceBasics {
   @_isc.useResult
   @override
   InstanceBasics copyWith({
+    String? app,
     Object? gitHubClientId = _Undefined,
     bool? emailSignIn,
     String? sourceCodeUrl,
@@ -150,6 +163,7 @@ class _InstanceBasicsImpl extends InstanceBasics {
     Object? backupLocation = _Undefined,
   }) {
     return InstanceBasics(
+      app: app ?? this.app,
       gitHubClientId: gitHubClientId is String?
           ? gitHubClientId
           : this.gitHubClientId,