feat(auto): add an Ecualizador browsable folder with preset selection

On-device feedback showed the equalizer's preset-cycling custom action
looked dead: many head units render custom actions icon-first, and a
monochrome icon cannot legibly encode "which of six presets" the way a
browsable list's text rows can.

This adds an "Ecualizador" folder to the car's browse tree, listing
"Desactivar" first, then the six factory presets by name, with the
currently-active one marked. Selecting a preset routes through the same
playFromMediaId seam every other browse-tree leaf already uses; picking
a preset while the equalizer is off turns it on and applies that preset.

Supersedes the earlier "no equalizer folder" rule (commit 2403da3),
which predated this feedback -- see decision auto/ecualizador-diseno.

The preset-cycling custom action still coexists with the folder in this
commit; it is removed in the next one.

lib/l10n/gen/app_localizations.dart

@@ -3068,6 +3068,12 @@ abstract class AppLocalizations {
   /// **'Preset: {preset}'**
   String eqCustomActionPresetLabel(String preset);
 
+  /// No description provided for @autoEqDisableOption.
+  ///
+  /// In es, this message translates to:
+  /// **'Desactivar'**
+  String get autoEqDisableOption;
+
   /// No description provided for @alarmCardVacationPausedBadge.
   ///
   /// In es, this message translates to: