feat(base,train): Train signs in with SimpleLogin, on ways back of its own (ADR 0020)

The forwarding page's table gains train-loopback (port 43825) and
train-scheme (app.halkyon.train://). Train's app had kept Todo's
loopback port 43824 from its shell; it now has its own.

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/lib/src/app/train_dependencies.dart

@@ -268,7 +268,7 @@ class TrainDependencies({
       // on by (halkyon_base's `simplelogin_callback.dart`): the scheme each
       // platform's project registers for the app, and Train's loopback port.
       callbackScheme: 'app.halkyon.train',
-      loopbackPort: 43824,
+      loopbackPort: 43825,
     );
     final instanceOperators = InstanceOperatorRepository(
       ServerpodInstanceOperatorGateway(client.modules.base),

apps/train/halkyon_train_flutter/test/app/train_app_test.dart

@@ -135,7 +135,7 @@ void main() {
             MemoryAccountKeyStore(),
             gateway,
             callbackScheme: 'app.halkyon.train',
-            loopbackPort: 43824,
+            loopbackPort: 43825,
           ),
           instanceOperators: InstanceOperatorRepository(
             FakeInstanceOperatorGateway(),

docs/adr/0020-sign-in-with-simplelogin.md

@@ -409,6 +409,14 @@ against app.simplelogin.io on 23 Oct:
     ```
 
   - **Released with the next changes** (over a release of its own).
+- **Halkyon Train (29 Oct 2026):** the forwarding page's table gains Train's ways back,
+  `train-loopback` on port 43825 and `train-scheme` (`app.halkyon.train://`), so that
+  Train signs up and in with SimpleLogin as Learn does; Train's app had taken Todo's port
+  43824 over with its shell, which would have sent a sign-in begun on one to the other on
+  the same desktop. The page is the register's, so Train signs in with SimpleLogin once
+  learn.halkyon.eu runs it; a development server of Train's needs the instance's
+  SimpleLogin app (`simpleLoginClientId`, `simpleLoginClientSecret`) and
+  `SIMPLELOGIN_CALLBACK_URL` naming learn.halkyon.eu's page.
 - **Also (23 Oct):** the developer's own account moves by adding SimpleLogin to it while
   signed in with GitHub; the instance's SimpleLogin app goes up for approval once
   halkyon.app/learn exists, before learn.halkyon.eu switches.

packages/halkyon_app/README.md

@@ -184,7 +184,7 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   `OidcBrowser`: SimpleLogin's page in the system's browser sheet, back by
   the app's scheme, or on Linux and Windows in the browser, back to the
   app's port on the loopback, as the instance's forwarding page names them
-  (`callbackScheme`, `loopbackPort`: Learn's 43823, Todo's 43824). Someone
+  (`callbackScheme`, `loopbackPort`: Learn's 43823, Todo's 43824, Train's 43825). Someone
   new creates an account (a handle, the one the instance suggests being
   free; the password twice, which makes the account's keys on the device;
   the recovery phrase shown once and three of its words typed back; nothing

packages/halkyon_app/lib/src/simplelogin/data/simplelogin_repository.dart

@@ -54,7 +54,7 @@ class SimpleLoginRepository(
 
   /// The app's own port on the loopback, which the browser comes back to on
   /// Linux and Windows, as the instance's forwarding page has it (Learn's
-  /// 43823, Todo's 43824).
+  /// 43823, Todo's 43824, Train's 43825).
   required final int loopbackPort,
 
   /// The platform the app runs on; tests name one.

packages/halkyon_base_server/lib/src/simplelogin/simplelogin_callback.dart

@@ -25,6 +25,8 @@ const simpleLoginReturns = {
   'learn-scheme': 'app.halkyon.learn://simplelogin-callback',
   'todo-loopback': 'http://127.0.0.1:43824/simplelogin-callback',
   'todo-scheme': 'app.halkyon.todo://simplelogin-callback',
+  'train-loopback': 'http://127.0.0.1:43825/simplelogin-callback',
+  'train-scheme': 'app.halkyon.train://simplelogin-callback',
 };
 
 final _returns = {

packages/halkyon_base_server/test/unit/simplelogin/simplelogin_callback_test.dart

@@ -25,6 +25,8 @@ void main() {
       'learn-scheme': 'app.halkyon.learn://simplelogin-callback',
       'todo-loopback': 'http://127.0.0.1:43824/simplelogin-callback',
       'todo-scheme': 'app.halkyon.todo://simplelogin-callback',
+      'train-loopback': 'http://127.0.0.1:43825/simplelogin-callback',
+      'train-scheme': 'app.halkyon.train://simplelogin-callback',
     });
   });
 

packages/halkyon_base_server/test/unit/simplelogin/simplelogin_config_test.dart

@@ -112,7 +112,7 @@ void main() {
       simpleLoginMarker('todo', SimpleLoginReturn.scheme),
       'todo-scheme',
     );
-    for (final app in ['learn', 'todo']) {
+    for (final app in ['learn', 'todo', 'train']) {
       for (final back in SimpleLoginReturn.values) {
         expect(
           simpleLoginReturns,