feat(auto): browse and play local music folders in Android Auto [size:exception]

Phase 1: pick a device folder via SAF (persisted grant, no new
permission), browse its nested subfolders/tracks as a 5th Android
Auto root folder (hidden until configured), and play tracks through
the existing pipeline (EQ, art rotation, cold-start-safe source).
No metadata/sort/filter/shuffle yet -- filename is the title, generic
rotating art is the placeholder; deferred to a follow-up phase.

Adds a new pluriwave/file_actions native method (listAudioChildren)
and an onActivityResult override in MainActivity for the SAF folder
picker -- both static-review-only, no Android build available here.

lib/l10n/app_de.arb

@@ -656,5 +656,18 @@
   "eqDeviceNameLabel": "Gerätename",
   "eqDeviceNameHint": "z. B. Wohnzimmer-Lautsprecher",
   "eqDeviceNameConfirm": "Speichern",
-  "eqDeviceConnected": "Verbunden"
+  "eqDeviceConnected": "Verbunden",
+  "localMusicSectionTitle": "Lokale Musik (Android Auto)",
+  "localMusicSectionDescription": "Wähle einen Ordner auf diesem Gerät aus, um dessen Audiodateien im Auto zu durchsuchen und abzuspielen.",
+  "localMusicFolderNotConfigured": "Kein Ordner ausgewählt",
+  "localMusicFolderTitle": "Ordner für lokale Musik",
+  "localMusicChoosePath": "Ordner auswählen",
+  "localMusicChangePath": "Ordner ändern",
+  "localMusicFolderUpdated": "Ordner für lokale Musik aktualisiert",
+  "localMusicFolderSaveError": "Ordner konnte nicht gespeichert werden: {error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }