feat(train): Home, Maps, Record, Groups and Profile in Learn's frame, washed in carrot (ADR 0030, D60)

Train's tabs are Strava's five where one screen with four tabs at its
top had held workouts, days, plans and routes: Home the workouts, Maps
the routes, Record recording and the interval plans, Groups a screen
saying they come later, Profile the days and the way to the account.
A device that doesn't record has no Record tab, its plans beside the
days in Profile.

The frame is halkyon_ui's NavigationFrame, driven by go_router as
Learn's: a workout's page opens in Home and a day's in Profile with the
tabs beside them; the route and plan editors and the recording cover
them, their content handed over as it is. Every tab washes down from the
top in carrot, the warm end of Train's icon, as Learn's in cobalt.

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/pubspec.yaml

@@ -33,6 +33,8 @@ dependencies:
   # The Jellyfin token in the keyring (ADR 0030, D47), as halkyon_app keeps
   # the account's keys.
   flutter_secure_storage: ^10.0.0
+  # The five tabs and the screens over them (ADR 0030, D60), as Learn's.
+  go_router: ^18.0.2
   halkyon_app:
     path: ../../../packages/halkyon_app
   # The device's half of signing in with OpenID Connect (ADR 0023): the