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>

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.