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/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(),