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/l10n/gen/app_localizations_en.dart

@@ -1228,4 +1228,26 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get streamUrlHint => 'https://stream.example.com:8000/radio';
+
+  @override
+  String get advancedEqSectionTitle => 'Advanced Equalization Options';
+
+  @override
+  String get advancedEqEnableToggle => 'Enable per-device EQ';
+
+  @override
+  String get advancedEqEnableToggleSubtitle =>
+      'Automatically apply a different EQ preset when your audio output changes.';
+
+  @override
+  String get advancedEqKnownDevicesTitle => 'Known audio devices';
+
+  @override
+  String get advancedEqKnownDevicesEmpty =>
+      'No audio devices recorded yet. Connect a device to get started.';
+
+  @override
+  String advancedEqDevicePresetLabel(Object presetName) {
+    return 'Preset: $presetName';
+  }
 }