fix(eq): split the equalizer body into 4 cards, add Restablecer a plano

Audit 11.10/11.8 (t4 lines 570-602): the prototype draws the explainer
banner, the band sliders, "Salida activa + Guardar como preset +
Restablecer a plano" and "Emisoras con ajuste propio" as 4 separate
cards. The build nested everything in one outer PluriGlassSurface, so
it read as a single merged block, and had no reset-to-flat action at
all.

Removes the outer wrapper from _CuerpoEcualizador. EcualizadorWidget
already draws its own card (design ADR-5, unchanged); the explainer
banner already has its own tinted background (unchanged); "Emisoras
con ajuste propio" now wraps itself in its own PluriGlassSurface; a
new _TarjetaSalidaYAcciones groups Salida activa with two tappable
rows — "Guardar como preset" (relocated from a floating end-aligned
button) and the new "Restablecer a plano", which applies
PresetEcualizador.flat via the existing EstadoEcualizador.cambiarPreset
— the same call the "Plano" preset chip already makes, not a new
capability.

The 5-slider band count, servicio_ecualizador.dart and
preset_ecualizador.dart are untouched.

lib/l10n/app_hi.arb

@@ -711,6 +711,7 @@
   "equalizerActiveOutputLabel": "सक्रिय आउटपुट",
   "equalizerBaseExplainer": "यह बेस इक्वलाइज़र है: यह उन सभी स्टेशन पर लागू होता है जिनकी अपनी सेटिंग नहीं है। किसी खास स्टेशन का अपना EQ उसकी प्लेबैक स्क्रीन से तय होता है और इस पर प्राथमिकता रखता है।",
   "equalizerSaveAsPresetAction": "प्रीसेट के रूप में सहेजें",
+  "equalizerResetToFlatAction": "समतल पर रीसेट करें",
   "equalizerSavePresetConfirm": "सहेजें",
   "equalizerSavePresetDialogTitle": "प्रीसेट के रूप में सहेजें",
   "equalizerSavePresetEmptyNameError": "प्रीसेट के लिए एक नाम डालें।",