feat(train): marked playlists kept on the phone, their tempo worked out (ADR 0030, D47)

Music's second part: the marked playlists' tracks are downloaded as
Jellyfin keeps them, on unmetered networks only, while the app is open
or resumes, and deleted once no marked playlist holds them. Each
track's tempo is worked out once on the phone: a BPM tag where the file
has one, else a minute decoded with MediaCodec and its spectral flux
autocorrelated (Tempo.kt, tested on the JVM). The music screen shows
what is kept and deletes it after asking.

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_de.arb

@@ -494,5 +494,12 @@
   "musicPlaylistsHint": "Train wählt aus den Titeln der Playlists, die du markierst, und behält sie auf diesem Telefon.",
   "musicNoPlaylists": "Auf diesem Server gibt es noch keine Musik-Playlists.",
   "musicRetry": "Erneut versuchen",
-  "musicTracks": "{count, plural, =1{1 Titel} other{{count} Titel}}"
+  "musicTracks": "{count, plural, =1{1 Titel} other{{count} Titel}}",
+  "musicKept": "Auf diesem Telefon",
+  "musicKeptWaiting": "{done} von {total} Titeln: der Rest wartet auf WLAN",
+  "musicKeptLoading": "Lädt: {done} von {total} Titeln",
+  "musicKeptDone": "{count} Titel, {size} MB, {tempo} mit Tempo",
+  "musicKeptDelete": "Löschen",
+  "musicKeptDeleteTitle": "Die Musik auf diesem Telefon löschen?",
+  "musicKeptDeleteText": "Alle Playlists werden abgewählt und ihre Titel von diesem Telefon gelöscht. Auf deinem Jellyfin-Server bleiben sie."
 }

apps/train/halkyon_train_flutter/lib/l10n/app_en.arb

@@ -621,5 +621,15 @@
   "musicNoPlaylists": "There are no music playlists on this server yet.",
   "musicRetry": "Try again",
   "musicTracks": "{count, plural, =1{1 track} other{{count} tracks}}",
-  "@musicTracks": {"placeholders": {"count": {"type": "int"}}}
+  "@musicTracks": {"placeholders": {"count": {"type": "int"}}},
+  "musicKept": "On this phone",
+  "musicKeptWaiting": "{done} of {total} tracks: the rest waits for Wi-Fi",
+  "@musicKeptWaiting": {"placeholders": {"done": {"type": "int"}, "total": {"type": "int"}}},
+  "musicKeptLoading": "Downloading: {done} of {total} tracks",
+  "@musicKeptLoading": {"placeholders": {"done": {"type": "int"}, "total": {"type": "int"}}},
+  "musicKeptDone": "{count} tracks, {size} MB, {tempo} with a tempo",
+  "@musicKeptDone": {"placeholders": {"count": {"type": "int"}, "size": {"type": "String", "example": "230.4"}, "tempo": {"type": "int"}}},
+  "musicKeptDelete": "Delete",
+  "musicKeptDeleteTitle": "Delete the music on this phone?",
+  "musicKeptDeleteText": "Every playlist is unmarked, and its tracks deleted from this phone. They stay on your Jellyfin server."
 }

apps/train/halkyon_train_flutter/lib/l10n/app_localizations.dart

@@ -2991,6 +2991,48 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'{count, plural, =1{1 track} other{{count} tracks}}'**
   String musicTracks(int count);
+
+  /// No description provided for @musicKept.
+  ///
+  /// In en, this message translates to:
+  /// **'On this phone'**
+  String get musicKept;
+
+  /// No description provided for @musicKeptWaiting.
+  ///
+  /// In en, this message translates to:
+  /// **'{done} of {total} tracks: the rest waits for Wi-Fi'**
+  String musicKeptWaiting(int done, int total);
+
+  /// No description provided for @musicKeptLoading.
+  ///
+  /// In en, this message translates to:
+  /// **'Downloading: {done} of {total} tracks'**
+  String musicKeptLoading(int done, int total);
+
+  /// No description provided for @musicKeptDone.
+  ///
+  /// In en, this message translates to:
+  /// **'{count} tracks, {size} MB, {tempo} with a tempo'**
+  String musicKeptDone(int count, String size, int tempo);
+
+  /// No description provided for @musicKeptDelete.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete'**
+  String get musicKeptDelete;
+
+  /// No description provided for @musicKeptDeleteTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete the music on this phone?'**
+  String get musicKeptDeleteTitle;
+
+  /// No description provided for @musicKeptDeleteText.
+  ///
+  /// In en, this message translates to:
+  /// **'Every playlist is unmarked, and its tracks deleted from this phone. They stay on your Jellyfin server.'**
+  String get musicKeptDeleteText;
 }
 
 class _AppLocalizationsDelegate

apps/train/halkyon_train_flutter/lib/l10n/app_localizations_de.dart

@@ -1781,4 +1781,32 @@ class AppLocalizationsDe extends AppLocalizations {
     );
     return '$_temp0';
   }
+
+  @override
+  String get musicKept => 'Auf diesem Telefon';
+
+  @override
+  String musicKeptWaiting(int done, int total) {
+    return '$done von $total Titeln: der Rest wartet auf WLAN';
+  }
+
+  @override
+  String musicKeptLoading(int done, int total) {
+    return 'Lädt: $done von $total Titeln';
+  }
+
+  @override
+  String musicKeptDone(int count, String size, int tempo) {
+    return '$count Titel, $size MB, $tempo mit Tempo';
+  }
+
+  @override
+  String get musicKeptDelete => 'Löschen';
+
+  @override
+  String get musicKeptDeleteTitle => 'Die Musik auf diesem Telefon löschen?';
+
+  @override
+  String get musicKeptDeleteText =>
+      'Alle Playlists werden abgewählt und ihre Titel von diesem Telefon gelöscht. Auf deinem Jellyfin-Server bleiben sie.';
 }

apps/train/halkyon_train_flutter/lib/l10n/app_localizations_en.dart

@@ -1769,6 +1769,34 @@ class AppLocalizationsEn extends AppLocalizations {
     );
     return '$_temp0';
   }
+
+  @override
+  String get musicKept => 'On this phone';
+
+  @override
+  String musicKeptWaiting(int done, int total) {
+    return '$done of $total tracks: the rest waits for Wi-Fi';
+  }
+
+  @override
+  String musicKeptLoading(int done, int total) {
+    return 'Downloading: $done of $total tracks';
+  }
+
+  @override
+  String musicKeptDone(int count, String size, int tempo) {
+    return '$count tracks, $size MB, $tempo with a tempo';
+  }
+
+  @override
+  String get musicKeptDelete => 'Delete';
+
+  @override
+  String get musicKeptDeleteTitle => 'Delete the music on this phone?';
+
+  @override
+  String get musicKeptDeleteText =>
+      'Every playlist is unmarked, and its tracks deleted from this phone. They stay on your Jellyfin server.';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).