fix(stations): replace the ringed circle thumbnail with a plain square

The prototype's station thumbnail is a plain 44-48px square, radius
11/12, with no ring or glow (t4 lines 84, 175, 227, 302, 614).
TarjetaEmisora's compact row variant instead painted a 58x58 circle
wrapped in a SweepGradient ring (magenta/cyan/coral) and a 22-blur
glow behind a 50x50 ClipRRect(18).

Replace it with a 48x48 ClipRRect(12) square. Update the loading
shimmer placeholder to match (was also a 58x58 circle block).

S6, Tier 1 visual-fidelity pass (audit id 2521).

lib/widgets/tarjeta_emisora.dart

@@ -8,7 +8,6 @@ import '../l10n/display_names.dart';
 import '../l10n/gen/app_localizations.dart';
 import '../modelos/emisora.dart';
 import '../tema/pluriwave_theme.dart';
-import '../tema/pluriwave_tokens.dart';
 import 'pluri_glass_surface.dart';
 import 'pluri_icon.dart';
 
@@ -183,35 +182,13 @@ class _TarjetaEmisoraState extends State<TarjetaEmisora> {
       ),
       child: Row(
         children: [
-          Stack(
-            alignment: Alignment.center,
-            children: [
-              Container(
-                width: 58,
-                height: 58,
-                decoration: BoxDecoration(
-                  shape: BoxShape.circle,
-                  gradient: SweepGradient(
-                    colors: [
-                      t.electricMagenta,
-                      PluriWaveTokens.brightCyan,
-                      t.warmCoral,
-                      t.electricMagenta,
-                    ],
-                  ),
-                  boxShadow: [
-                    BoxShadow(
-                      color: t.glowColor.withValues(alpha: 0.24),
-                      blurRadius: 22,
-                    ),
-                  ],
-                ),
-              ),
-              ClipRRect(
-                borderRadius: BorderRadius.circular(18),
-                child: SizedBox(width: 50, height: 50, child: _logo(24)),
-              ),
-            ],
+          // S6 (Tier 1 visual fidelity): the prototype's station thumbnail
+          // is a plain square, radius 11/12, with NO ring or glow (t4 lines
+          // 84, 175, 227, 302, 614) — was a 58x58 circle wrapped in a
+          // SweepGradient ring and a 22-blur glow.
+          ClipRRect(
+            borderRadius: BorderRadius.circular(12),
+            child: SizedBox(width: 48, height: 48, child: _logo(22)),
           ),
           SizedBox(width: t.spacingSm),
           Expanded(
@@ -454,7 +431,9 @@ class TarjetaEmisoraShimmer extends StatelessWidget {
         esCompacta
             ? Row(
               children: [
-                bloque(width: 58, height: 58, shape: BoxShape.circle),
+                // S6: matches the real square thumbnail (48x48, radius 12)
+                // — was a 58x58 circle block.
+                bloque(width: 48, height: 48, radius: 12),
                 const SizedBox(width: 10),
                 Expanded(
                   child: Column(