feat(eq): add per-device equalizer with 4-level preset resolution

Introduce multi-device EQ support allowing each audio output device
(built-in speaker, wired, USB, individual Bluetooth by MAC) to have
its own equalizer preset, combined with existing per-station presets
for a full station×device matrix.

- Add DispositivoAudio model and ServicioDispositivoAudio interface
- Add Android platform channel (AudioDeviceCallback) for device detection
- Add iOS AudioDevicesPlugin (AVAudioSession route tracking)
- Extend ServicioEcualizador with device and matrix persistence keys
- Implement 4-level resolution: matrix > station > device > global
- Add advanced EQ settings section with feature toggle (off by default)
- Extend export/import to v3 with backward compatibility
- 184 tests passing, zero analyzer issues

lib/modelos/dispositivo_audio.dart

@@ -0,0 +1,47 @@
+/// Audio device types detected via platform channel.
+enum TipoDispositivo {
+  /// Built-in speaker — id: "builtin_speaker"
+  altavozInterno,
+
+  /// Wired headset or headphones — id: "wired_headset"
+  auricularesCable,
+
+  /// Bluetooth A2DP device — id: `"bt_a2dp:<MAC>"`
+  bluetoothA2dp,
+
+  /// USB audio device — id: `"usb_headset:<address>"`
+  usbAudio,
+
+  /// Unrecognized device type.
+  desconocido,
+}
+
+/// Represents an audio output device detected on the current platform.
+///
+/// Equality is based on [id] only so that two instances referring to the
+/// same physical device compare as equal regardless of display name.
+class DispositivoAudio {
+  final String id;
+  final TipoDispositivo tipo;
+  final String nombre;
+
+  const DispositivoAudio({
+    required this.id,
+    required this.tipo,
+    required this.nombre,
+  });
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+    if (other is! DispositivoAudio) return false;
+    return id == other.id;
+  }
+
+  @override
+  int get hashCode => id.hashCode;
+
+  @override
+  String toString() =>
+      'DispositivoAudio(id: $id, tipo: $tipo, nombre: $nombre)';
+}