feat(train): Halkyon Train's shell, Todo's copied and renamed (ADR 0030, D12)

apps/train with halkyon_train_server, _client and _flutter, as Todo's:
halkyon_base configured by trainBase(), its own register alone and a
member of the instance's where the operator names one, the app on
halkyon_app's gates with Train's frame, account sheet and About, texts
in British English and German that speak of workouts, app.halkyon.train
on Android, iOS, Linux, macOS and Windows.

Where it differs from Todo: development ports of its own (8180 to 8190),
so it runs beside Learn's server; one initial migration with the schema
Todo's migrations end at (the same tables, compared line by line), not
Todo's seventeen; and Todo's icon as a placeholder until the icon tool
of ADR 0031 draws Train's bike. The workspace, the licences, the READMEs
and AGENTS.md name it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

README.md

@@ -9,8 +9,9 @@ servers and everything needed to host an instance are published.
 |---|---|---|
 | Halkyon Learn | Creating, sharing and studying study material, flashcards scheduled with FSRS | [`apps/learn/`](apps/learn/README.md), at learn.halkyon.eu |
 | Halkyon Todo | To-dos; for now a shell on the shared packages: sign-in, instances, the account | [`apps/todo/`](apps/todo/) |
+| Halkyon Train | Running, cycling and rowing (ADR 0030); for now a shell on the shared packages | [`apps/train/`](apps/train/) |
 
-Halkyon Train and Halkyon Meditate are to follow.
+Halkyon Meditate is to follow.
 
 ## Layout