refactor(state): extract export/import service and equalizer state from EstadoRadio

- New ServicioExportImport owns the v2 backup envelope, pretty JSON encode and graceful decode; byte-compatible with existing exports, locked by a round-trip test
- pantalla_ajustes delegates backup serialization to the service (inline jsonDecode/jsonEncode removed)
- New EstadoEcualizador ChangeNotifier owns all EQ state and persistence (principal/current/per-station presets, active flag), exposed via its own provider so EQ changes no longer rebuild EstadoRadio consumers
- EstadoRadio slims down ~210 lines and keeps 15 delegating compat members marked TODO(S4b) for the next slice to remove
- Player EQ toggle rewired to the new provider to avoid going stale
- 4 new tests (103 total green), flutter analyze clean

lib/app.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
+import 'estado/estado_ecualizador.dart';
 import 'estado/estado_radio.dart';
 import 'estado/estado_alarmas.dart';
 import 'estado/estado_idioma.dart';
@@ -35,6 +36,12 @@ class PluriWaveApp extends StatelessWidget {
     return MultiProvider(
       providers: [
         ChangeNotifierProvider(create: (_) => EstadoRadio(prefs: prefs)),
+        // EQ notifier (S4-R1). Created and disposed by EstadoRadio during
+        // the S4 transition; this provider only exposes the instance, so it
+        // declares no dispose callback.
+        ListenableProvider<EstadoEcualizador>(
+          create: (context) => context.read<EstadoRadio>().ecualizador,
+        ),
         ChangeNotifierProvider(create: (_) => EstadoAlarmas(prefs: prefs)),
         ChangeNotifierProvider(
           create: (_) => EstadoIdioma(sharedPreferences: prefs),