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(
+          // S6 (Tier 1 visual fidelity): the prototype's station thumbnail
-            alignment: Alignment.center,
+          // is a plain square, radius 11/12, with NO ring or glow (t4 lines
-            children: [
+          // 84, 175, 227, 302, 614) — was a 58x58 circle wrapped in a
-              Container(
+          // SweepGradient ring and a 22-blur glow.
-                width: 58,
+          ClipRRect(
-                height: 58,
+            borderRadius: BorderRadius.circular(12),
-                decoration: BoxDecoration(
+            child: SizedBox(width: 48, height: 48, child: _logo(22)),
-                  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)),
-              ),
-            ],
           ),
           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(

test/widgets/tarjeta_emisora_thumbnail_test.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/estado/estado_radio.dart';
+import 'package:pluriwave/l10n/gen/app_localizations.dart';
+import 'package:pluriwave/widgets/tarjeta_emisora.dart';
+import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import '../helpers/fakes.dart';
+
+/// S6 (Tier 1 visual fidelity): the prototype's station thumbnail is a
+/// plain 44-48px square, radius 11/12, with NO ring or glow (t4 lines 84
+/// `44px/radius 11`, 175 `46px/radius 12`, 227 `48px/radius 12`, 302
+/// `48px/radius 12`, 614 `44px/radius 12`). The compact row variant used to
+/// paint a 58x58 circle with a `SweepGradient` ring and a 22-blur glow
+/// behind a 50x50 `ClipRRect(18)`.
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  Widget host(Widget child) {
+    final estado = EstadoRadio(
+      audio: FakeServicioAudio(),
+      favoritos: FakeServicioFavoritos(),
+      radio: FakeServicioRadio(),
+      servicioEcualizador: FakeServicioEcualizador(),
+      iniciarAutomaticamente: false,
+    );
+    addTearDown(estado.dispose);
+    return ChangeNotifierProvider<EstadoRadio>.value(
+      value: estado,
+      child: MaterialApp(
+        locale: const Locale('en'),
+        localizationsDelegates: AppLocalizations.localizationsDelegates,
+        supportedLocales: AppLocalizations.supportedLocales,
+        home: Scaffold(body: Center(child: child)),
+      ),
+    );
+  }
+
+  bool isRingContainer(Widget w) =>
+      w is Container &&
+      w.decoration is BoxDecoration &&
+      (w.decoration as BoxDecoration).shape == BoxShape.circle &&
+      (w.decoration as BoxDecoration).gradient is SweepGradient;
+
+  testWidgets('esCompacta (row) thumbnail is a 48x48 square, radius 12, no '
+      'ring/glow', (tester) async {
+    await tester.pumpWidget(
+      host(
+        SizedBox(
+          width: 320,
+          child: TarjetaEmisora(
+            emisora: emisoraDemo(uuid: 'row', nombre: 'Row FM'),
+            esCompacta: true,
+          ),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    expect(find.byWidgetPredicate(isRingContainer), findsNothing);
+
+    // Not `.first` — the outer PluriGlassSurface also renders its own
+    // ClipRRect (radius 14, the row surface radius from S4), so the
+    // thumbnail's own clip must be found by its distinct radius instead of
+    // tree order.
+    final thumbnailClip = find.byWidgetPredicate(
+      (w) => w is ClipRRect && w.borderRadius == BorderRadius.circular(12),
+    );
+    expect(thumbnailClip, findsOneWidget);
+    expect(tester.getSize(thumbnailClip), const Size(48, 48));
+  });
+
+  testWidgets('esCompacta shimmer placeholder is a square block, not a '
+      'circle', (tester) async {
+    await tester.pumpWidget(
+      host(const TarjetaEmisoraShimmer(esCompacta: true)),
+    );
+    await tester.pump();
+
+    expect(find.byWidgetPredicate(isRingContainer), findsNothing);
+    final circles = tester
+        .widgetList<Container>(find.byType(Container))
+        .where(
+          (c) =>
+              c.decoration is BoxDecoration &&
+              (c.decoration as BoxDecoration).shape == BoxShape.circle,
+        );
+    expect(
+      circles,
+      isEmpty,
+      reason: 'the real thumbnail is square now — its shimmer must match',
+    );
+  });
+}