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.

lib/l10n/gen/app_localizations.dart

@@ -3049,6 +3049,24 @@ abstract class AppLocalizations {
   /// In es, this message translates to:
   /// **'Empezar a escuchar'**
   String get welcomeCtaLabel;
+
+  /// No description provided for @eqCustomActionEnableLabel.
+  ///
+  /// In es, this message translates to:
+  /// **'Activar ecualizador'**
+  String get eqCustomActionEnableLabel;
+
+  /// No description provided for @eqCustomActionDisableLabel.
+  ///
+  /// In es, this message translates to:
+  /// **'Desactivar ecualizador'**
+  String get eqCustomActionDisableLabel;
+
+  /// No description provided for @eqCustomActionPresetLabel.
+  ///
+  /// In es, this message translates to:
+  /// **'Preset: {preset}'**
+  String eqCustomActionPresetLabel(String preset);
 }
 
 class _AppLocalizationsDelegate

lib/l10n/gen/app_localizations_ar.dart

@@ -1676,4 +1676,15 @@ class AppLocalizationsAr extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'ابدأ الاستماع';
+
+  @override
+  String get eqCustomActionEnableLabel => 'تفعيل الموازن';
+
+  @override
+  String get eqCustomActionDisableLabel => 'إيقاف الموازن';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'الإعداد المسبق: $preset';
+  }
 }

lib/l10n/gen/app_localizations_bn.dart

@@ -1685,4 +1685,15 @@ class AppLocalizationsBn extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'শোনা শুরু করুন';
+
+  @override
+  String get eqCustomActionEnableLabel => 'ইকুয়ালাইজার চালু করুন';
+
+  @override
+  String get eqCustomActionDisableLabel => 'ইকুয়ালাইজার বন্ধ করুন';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'প্রিসেট: $preset';
+  }
 }

lib/l10n/gen/app_localizations_de.dart

@@ -1698,4 +1698,15 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Jetzt hören';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Equalizer aktivieren';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Equalizer deaktivieren';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Voreinstellung: $preset';
+  }
 }

lib/l10n/gen/app_localizations_en.dart

@@ -1678,4 +1678,15 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Start listening';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Enable equalizer';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Disable equalizer';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Preset: $preset';
+  }
 }

lib/l10n/gen/app_localizations_es.dart

@@ -1692,4 +1692,15 @@ class AppLocalizationsEs extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Empezar a escuchar';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Activar ecualizador';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Desactivar ecualizador';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Preset: $preset';
+  }
 }

lib/l10n/gen/app_localizations_fr.dart

@@ -1701,4 +1701,15 @@ class AppLocalizationsFr extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Commencer à écouter';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Activer l\'égaliseur';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Désactiver l\'égaliseur';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Préréglage : $preset';
+  }
 }

lib/l10n/gen/app_localizations_hi.dart

@@ -1680,4 +1680,15 @@ class AppLocalizationsHi extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'सुनना शुरू करें';
+
+  @override
+  String get eqCustomActionEnableLabel => 'इक्वलाइज़र चालू करें';
+
+  @override
+  String get eqCustomActionDisableLabel => 'इक्वलाइज़र बंद करें';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'प्रीसेट: $preset';
+  }
 }

lib/l10n/gen/app_localizations_id.dart

@@ -1688,4 +1688,15 @@ class AppLocalizationsId extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Mulai mendengarkan';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Aktifkan equalizer';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Nonaktifkan equalizer';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Prasetel: $preset';
+  }
 }

lib/l10n/gen/app_localizations_it.dart

@@ -1700,4 +1700,15 @@ class AppLocalizationsIt extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Inizia ad ascoltare';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Attiva equalizzatore';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Disattiva equalizzatore';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Preset attivo: $preset';
+  }
 }

lib/l10n/gen/app_localizations_ja.dart

@@ -1631,4 +1631,15 @@ class AppLocalizationsJa extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => '聴き始める';
+
+  @override
+  String get eqCustomActionEnableLabel => 'イコライザーをオンにする';
+
+  @override
+  String get eqCustomActionDisableLabel => 'イコライザーをオフにする';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'プリセット: $preset';
+  }
 }

lib/l10n/gen/app_localizations_pt.dart

@@ -1688,4 +1688,15 @@ class AppLocalizationsPt extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Começar a ouvir';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Ativar equalizador';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Desativar equalizador';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Predefinição: $preset';
+  }
 }

lib/l10n/gen/app_localizations_ru.dart

@@ -1692,4 +1692,15 @@ class AppLocalizationsRu extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => 'Начать слушать';
+
+  @override
+  String get eqCustomActionEnableLabel => 'Включить эквалайзер';
+
+  @override
+  String get eqCustomActionDisableLabel => 'Выключить эквалайзер';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return 'Пресет: $preset';
+  }
 }

lib/l10n/gen/app_localizations_zh.dart

@@ -1623,4 +1623,15 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get welcomeCtaLabel => '开始收听';
+
+  @override
+  String get eqCustomActionEnableLabel => '启用均衡器';
+
+  @override
+  String get eqCustomActionDisableLabel => '关闭均衡器';
+
+  @override
+  String eqCustomActionPresetLabel(String preset) {
+    return '预设：$preset';
+  }
 }