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."
 }