fix(devices): cache platform device names, dedupe placeholder ids, purge collided EQ entries

Dart half of bt-device-identity. EstadoEcualizador now caches each
device's platform-reported name in memory so the settings screen
shows the device's own Bluetooth name instead of its raw id when no
custom rename exists, and skips auto-creating preset entries for the
composite-placeholder sentinel. Enabling multi-device EQ triggers the
Bluetooth permission request through the new channel contract. A
flag-guarded one-time migration purges only entries keyed by the
exact literal placeholder id from the three per-device preference
maps, since those collided entries cannot be attributed to a device.

Work unit 2/2 of bt-device-identity (Dart state + migration).

test/servicios/servicio_dispositivo_audio_toggle_test.dart

@@ -24,6 +24,9 @@ class NullServicioDispositivoAudio extends ServicioDispositivoAudio {
     );
   }
 
+  @override
+  Future<bool> solicitarPermisoBluetooth() async => true;
+
   @override
   Future<void> dispose() async {}
 }