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/lib/l10n/app_localizations.dart

@@ -2835,6 +2835,162 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Heading up'**
   String get recordingHeadingUp;
+
+  /// The music screen's title, and its tile in the account sheet.
+  ///
+  /// In en, this message translates to:
+  /// **'Music'**
+  String get musicTitle;
+
+  /// Under Music in the account sheet.
+  ///
+  /// In en, this message translates to:
+  /// **'Jellyfin playlists to run to'**
+  String get musicHint;
+
+  /// No description provided for @musicIntro.
+  ///
+  /// In en, this message translates to:
+  /// **'Train plays music from your Jellyfin server, choosing tracks whose beat fits your steps. Your account and music stay on this phone.'**
+  String get musicIntro;
+
+  /// The label of the server's address.
+  ///
+  /// In en, this message translates to:
+  /// **'Jellyfin server'**
+  String get musicServer;
+
+  /// No description provided for @musicConnect.
+  ///
+  /// In en, this message translates to:
+  /// **'Connect'**
+  String get musicConnect;
+
+  /// No description provided for @musicBadAddress.
+  ///
+  /// In en, this message translates to:
+  /// **'Enter the server\'s address, such as jellyfin.example.org.'**
+  String get musicBadAddress;
+
+  /// No description provided for @musicUnreachable.
+  ///
+  /// In en, this message translates to:
+  /// **'The server can\'t be reached. Check the address and the connection.'**
+  String get musicUnreachable;
+
+  /// No description provided for @musicNotJellyfin.
+  ///
+  /// In en, this message translates to:
+  /// **'There is no Jellyfin server at this address.'**
+  String get musicNotJellyfin;
+
+  /// No description provided for @musicRefused.
+  ///
+  /// In en, this message translates to:
+  /// **'Jellyfin refused. Check the user name and password.'**
+  String get musicRefused;
+
+  /// No description provided for @musicCodeExpired.
+  ///
+  /// In en, this message translates to:
+  /// **'The code has expired.'**
+  String get musicCodeExpired;
+
+  /// No description provided for @musicFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Jellyfin answered with an error. Try again later.'**
+  String get musicFailed;
+
+  /// No description provided for @musicQuickConnect.
+  ///
+  /// In en, this message translates to:
+  /// **'To sign in to {server}, enter this code in Jellyfin on another device, under Settings, Quick Connect.'**
+  String musicQuickConnect(String server);
+
+  /// No description provided for @musicNewCode.
+  ///
+  /// In en, this message translates to:
+  /// **'New code'**
+  String get musicNewCode;
+
+  /// No description provided for @musicBack.
+  ///
+  /// In en, this message translates to:
+  /// **'Back'**
+  String get musicBack;
+
+  /// No description provided for @musicUsePassword.
+  ///
+  /// In en, this message translates to:
+  /// **'Use a password'**
+  String get musicUsePassword;
+
+  /// No description provided for @musicPasswordIntro.
+  ///
+  /// In en, this message translates to:
+  /// **'Sign in to {server} with your Jellyfin user name and password.'**
+  String musicPasswordIntro(String server);
+
+  /// No description provided for @musicUserName.
+  ///
+  /// In en, this message translates to:
+  /// **'User name'**
+  String get musicUserName;
+
+  /// No description provided for @musicPassword.
+  ///
+  /// In en, this message translates to:
+  /// **'Password'**
+  String get musicPassword;
+
+  /// No description provided for @musicSignIn.
+  ///
+  /// In en, this message translates to:
+  /// **'Sign in'**
+  String get musicSignIn;
+
+  /// No description provided for @musicSignedInAs.
+  ///
+  /// In en, this message translates to:
+  /// **'Signed in as {name}'**
+  String musicSignedInAs(String name);
+
+  /// No description provided for @musicSignOut.
+  ///
+  /// In en, this message translates to:
+  /// **'Sign out'**
+  String get musicSignOut;
+
+  /// No description provided for @musicPlaylists.
+  ///
+  /// In en, this message translates to:
+  /// **'Playlists for running'**
+  String get musicPlaylists;
+
+  /// No description provided for @musicPlaylistsHint.
+  ///
+  /// In en, this message translates to:
+  /// **'Train chooses from the tracks of the playlists you mark, and keeps them on this phone.'**
+  String get musicPlaylistsHint;
+
+  /// No description provided for @musicNoPlaylists.
+  ///
+  /// In en, this message translates to:
+  /// **'There are no music playlists on this server yet.'**
+  String get musicNoPlaylists;
+
+  /// No description provided for @musicRetry.
+  ///
+  /// In en, this message translates to:
+  /// **'Try again'**
+  String get musicRetry;
+
+  /// No description provided for @musicTracks.
+  ///
+  /// In en, this message translates to:
+  /// **'{count, plural, =1{1 track} other{{count} tracks}}'**
+  String musicTracks(int count);
 }
 
 class _AppLocalizationsDelegate