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_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 = {