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