fix(escuchar): restore the prototype's 132px hero artwork

Audit 1.3: the hero art shipped at 84 where the prototype draws 132
(t4 line 56), with a 24 corner radius rather than the shared radiusMd.

Adds a dimension guard. Every existing test on this screen asserts
behaviour only, which is why an artwork 36% too small went unnoticed.

lib/pantallas/pantalla_inicio.dart

@@ -307,12 +307,15 @@ class _ArteEscuchar extends StatelessWidget {
 
   final Emisora emisora;
 
-  static const _lado = 84.0;
+  /// Audit 1.3: the prototype's Escuchar hero art is 132 (t4 line 56), and
+  /// its corner radius is 24 rather than the shared `radiusMd`.
+  static const _lado = 132.0;
+  static const _radio = 24.0;
 
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
-    final radius = BorderRadius.circular(context.pluriTokens.radiusMd);
+    final radius = BorderRadius.circular(_radio);
     return PluriGlassSurface(
       padding: EdgeInsets.zero,
       borderRadius: radius,