feat(auto): equalizer enable/disable and preset cycling from the car

Expose the equalizer's on/off toggle and preset choice as PlaybackStateCompat
custom actions on the now-playing screen. The redesign's removal of the
in-car equalizer FOLDER from the browse tree stays as-is (2403da3) -- this
is a different surface (playback screen custom actions, not a browse
folder) and does not reintroduce it.

Deliberately just 2 actions -- an on/off toggle plus a cycling preset
action, not one action per preset -- since Android Auto only surfaces a
limited number of custom actions. Both reuse the existing
setEcualizadorActivo/aplicarPreset entry points (the same ones
EstadoEcualizador's phone settings screen uses), so a car tap and a phone
tap behave identically and both keep the action labels in sync. Reuses the
bundled ic_stat_pluriwave drawable (the notification's own equalizer-bars
icon) -- zero new native assets. The 5-band constraint is untouched.

New pure, unit-tested functions in servicio_audio.dart: presetSiguiente,
nombrePresetVisible, controlesEcualizadorPersonalizados. New ARB keys
(eqCustomActionEnableLabel/DisableLabel/PresetLabel) across all 13 locales,
regenerated via flutter gen-l10n.

test/servicios/servicio_audio_eq_custom_actions_test.dart

@@ -0,0 +1,189 @@
+import 'dart:ui' show Locale;
+
+import 'package:audio_service/audio_service.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/l10n/gen/app_localizations.dart';
+import 'package:pluriwave/modelos/preset_ecualizador.dart';
+import 'package:pluriwave/servicios/servicio_audio.dart';
+
+/// Item 4 (Android Auto: equalizer custom actions) — the pure, handler-
+/// independent half of the fix. `PluriWaveAudioHandler` cannot be
+/// instantiated in unit tests (a real `just_audio.AudioPlayer` requires
+/// platform MethodChannels), so the preset-cycling decision, the preset-name
+/// localization and the `MediaControl` list construction are extracted as
+/// pure top-level functions here. The handler's own `customAction` dispatch
+/// and `playbackState` wiring are static-review-only, same as the existing
+/// EQ re-apply/session-id wiring.
+void main() {
+  final l10n = lookupAppLocalizations(const Locale('es'));
+
+  group('presetSiguiente (item 4 — cycling presets)', () {
+    test('advances to the next preset in order', () {
+      expect(
+        presetSiguiente(PresetEcualizador.flat),
+        PresetEcualizador.rock,
+      );
+      expect(
+        presetSiguiente(PresetEcualizador.rock),
+        PresetEcualizador.pop,
+      );
+    });
+
+    test('wraps around after the last preset', () {
+      expect(
+        presetSiguiente(PresetEcualizador.presets.last),
+        PresetEcualizador.presets.first,
+      );
+    });
+
+    test(
+      'an unknown/custom preset (e.g. a user-tweaked "Personalizado" band '
+      'set) starts from the FIRST preset instead of throwing',
+      () {
+        final personalizado = PresetEcualizador(
+          nombre: 'Personalizado',
+          bandas: [1.0, 2.0, 3.0, 4.0, 5.0],
+        );
+
+        expect(
+          presetSiguiente(personalizado),
+          PresetEcualizador.presets.first,
+        );
+      },
+    );
+
+    test('respects an injected presets list instead of the default 6', () {
+      final propios = [PresetEcualizador.jazz, PresetEcualizador.voz];
+
+      expect(
+        presetSiguiente(PresetEcualizador.jazz, presets: propios),
+        PresetEcualizador.voz,
+      );
+      expect(
+        presetSiguiente(PresetEcualizador.voz, presets: propios),
+        PresetEcualizador.jazz,
+      );
+    });
+  });
+
+  group('nombrePresetVisible (item 4)', () {
+    test('maps every factory preset name to its localized ARB string', () {
+      expect(nombrePresetVisible(l10n, 'Flat'), l10n.equalizerPresetFlat);
+      expect(nombrePresetVisible(l10n, 'Rock'), l10n.equalizerPresetRock);
+      expect(nombrePresetVisible(l10n, 'Pop'), l10n.equalizerPresetPop);
+      expect(
+        nombrePresetVisible(l10n, 'Bass Boost'),
+        l10n.equalizerPresetBassBoost,
+      );
+      expect(nombrePresetVisible(l10n, 'Jazz'), l10n.equalizerPresetJazz);
+      expect(nombrePresetVisible(l10n, 'Voz'), l10n.equalizerPresetVoice);
+      expect(
+        nombrePresetVisible(l10n, 'Personalizado'),
+        l10n.equalizerPresetCustom,
+      );
+    });
+
+    test('an unrecognized name falls through verbatim', () {
+      expect(nombrePresetVisible(l10n, 'Mi Preset Guardado'), 'Mi Preset Guardado');
+    });
+  });
+
+  group('controlesEcualizadorPersonalizados (item 4)', () {
+    test('empty when the equalizer is not available on this device', () {
+      final controles = controlesEcualizadorPersonalizados(
+        disponible: false,
+        activo: true,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+      );
+
+      expect(controles, isEmpty);
+    });
+
+    test(
+      'exactly 2 custom actions when available: on/off toggle + preset '
+      'cycle -- Android Auto shows a limited number of custom actions, so '
+      'this is deliberately NOT one action per preset',
+      () {
+        final controles = controlesEcualizadorPersonalizados(
+          disponible: true,
+          activo: true,
+          presetActual: PresetEcualizador.rock,
+          l10n: l10n,
+        );
+
+        expect(controles, hasLength(2));
+        expect(controles.every((c) => c.action == MediaAction.custom), isTrue);
+      },
+    );
+
+    test('toggle label reflects ON -> shows "disable" action', () {
+      final controles = controlesEcualizadorPersonalizados(
+        disponible: true,
+        activo: true,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+      );
+      final toggle = controles.firstWhere(
+        (c) => c.customAction?.name == accionEqToggle,
+      );
+
+      expect(toggle.label, l10n.eqCustomActionDisableLabel);
+    });
+
+    test('toggle label reflects OFF -> shows "enable" action', () {
+      final controles = controlesEcualizadorPersonalizados(
+        disponible: true,
+        activo: false,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+      );
+      final toggle = controles.firstWhere(
+        (c) => c.customAction?.name == accionEqToggle,
+      );
+
+      expect(toggle.label, l10n.eqCustomActionEnableLabel);
+    });
+
+    test('preset-cycle label shows the CURRENT preset localized name', () {
+      final controles = controlesEcualizadorPersonalizados(
+        disponible: true,
+        activo: true,
+        presetActual: PresetEcualizador.jazz,
+        l10n: l10n,
+      );
+      final ciclo = controles.firstWhere(
+        (c) => c.customAction?.name == accionEqPresetSiguiente,
+      );
+
+      expect(
+        ciclo.label,
+        l10n.eqCustomActionPresetLabel(l10n.equalizerPresetJazz),
+      );
+    });
+
+    test('both actions reuse the bundled notification drawable (zero new '
+        'native assets)', () {
+      final controles = controlesEcualizadorPersonalizados(
+        disponible: true,
+        activo: true,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+      );
+
+      expect(
+        controles.every((c) => c.androidIcon == 'drawable/ic_stat_pluriwave'),
+        isTrue,
+      );
+    });
+  });
+
+  group('action name constants (item 4 — collision-free with car-tree ids)', () {
+    test('names are non-empty, distinct, and do not collide with any '
+        'existing browse-tree media-id prefix', () {
+      expect(accionEqToggle, isNotEmpty);
+      expect(accionEqPresetSiguiente, isNotEmpty);
+      expect(accionEqToggle, isNot(equals(accionEqPresetSiguiente)));
+    });
+  });
+}