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/app_es.arb

@@ -579,5 +579,16 @@
         "type": "int"
       }
     }
+  },
+  "advancedEqSectionTitle": "Opciones avanzadas de ecualización",
+  "advancedEqEnableToggle": "Activar EQ por dispositivo",
+  "advancedEqEnableToggleSubtitle": "Aplica automáticamente un preset de EQ distinto al cambiar la salida de audio.",
+  "advancedEqKnownDevicesTitle": "Dispositivos de audio conocidos",
+  "advancedEqKnownDevicesEmpty": "No hay dispositivos registrados. Conectá uno para empezar.",
+  "advancedEqDevicePresetLabel": "Preset: {presetName}",
+  "@advancedEqDevicePresetLabel": {
+    "placeholders": {
+      "presetName": {}
+    }
   }
 }