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/app_de.arb

@@ -830,5 +830,15 @@
   "welcomeHeadline": "Deine Welt, live",
   "yourStationsTitle": "Deine Sender",
   "nowListeningLabel": "Läuft gerade",
-  "popularNowTitle": "Jetzt beliebt"
+  "popularNowTitle": "Jetzt beliebt",
+  "eqCustomActionEnableLabel": "Equalizer aktivieren",
+  "eqCustomActionDisableLabel": "Equalizer deaktivieren",
+  "eqCustomActionPresetLabel": "Voreinstellung: {preset}",
+  "@eqCustomActionPresetLabel": {
+    "placeholders": {
+      "preset": {
+        "type": "String"
+      }
+    }
+  }
 }