feat(train): signing in to Jellyfin and marking playlists (ADR 0030, D47)

Music's decisions as D47, and its first part: the music screen from the
account sheet, on Android, signs in to the person's Jellyfin with Quick
Connect, else a password, and lists their playlists of music to mark
for running. The token is in the keyring, the server and the marks in
the person's music.json, nothing on Train's server.

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

@@ -30,6 +30,9 @@ dependencies:
   # vector tiles of the instance's source (OpenFreeMap without a setting, D22),
   # and its coordinates.
   flutter_map: ^8.3.2
+  # The Jellyfin token in the keyring (ADR 0030, D47), as halkyon_app keeps
+  # the account's keys.
+  flutter_secure_storage: ^10.0.0
   halkyon_app:
     path: ../../../packages/halkyon_app
   # The device's half of signing in with OpenID Connect (ADR 0023): the