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>

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.