fix(widgets): share station-art fallback across every surface

TarjetaEmisora had the only good fallback for a station with no artwork --
a deterministic pick from 4 bundled illustrations with a gradient/glyph
last resort. FilaEmisoraPlana's flat rows, the Escuchar hero, the "Tus
emisoras" grid cell, the mini player and the full player each had their
own, separate, flat primaryContainer square instead.

Extract the good fallback into PluriStationArtFallback and use it from
every one of those call sites. The selection formula (asset order,
codeUnits-sum modulo) is preserved exactly, since navegacion_auto.dart
mirrors the same formula independently for Android Auto's own drawable
rotation.

test/pantallas/pantalla_inicio_test.dart

@@ -11,6 +11,7 @@ import 'package:pluriwave/l10n/gen/app_localizations.dart';
 import 'package:pluriwave/modelos/emisora.dart';
 import 'package:pluriwave/pantallas/pantalla_inicio.dart';
 import 'package:pluriwave/widgets/pluri_root_header.dart';
+import 'package:pluriwave/widgets/pluri_station_art_fallback.dart';
 import 'package:pluriwave/widgets/visualizador_audio.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -698,6 +699,76 @@ void main() {
       );
     },
   );
+
+  testWidgets('issue 6 (feedback-pruebas): the Escuchar hero shows the shared '
+      'PluriStationArtFallback for a station with no favicon, not a flat '
+      'coloured square', (tester) async {
+    _setLargeSurfaceSize(tester);
+    final favoritos = FakeServicioFavoritos();
+    final estado = EstadoRadio(
+      audio: FakeServicioAudio(),
+      favoritos: favoritos,
+      radio: FakeServicioRadio(),
+      servicioEcualizador: FakeServicioEcualizador(),
+      servicioGrabacion: FakeServicioGrabacionRadio(),
+      resolverArchivoCustom: _archivoCustomVacio,
+      iniciarAutomaticamente: false,
+    );
+    addTearDown(estado.dispose);
+    await tester.runAsync(estado.inicializar);
+    final sonando = emisoraDemo(uuid: 'sin-arte', nombre: 'Sin Arte');
+    await tester.runAsync(() => estado.reproducir(sonando));
+
+    await tester.pumpWidget(
+      _conProviders(estado, _testApp(const PantallaInicio())),
+    );
+    await _pumpBounded(tester);
+
+    expect(
+      find.byType(PluriStationArtFallback),
+      findsWidgets,
+      reason:
+          'issue 6: the hero must reach the shared fallback, not its own '
+          'flat primaryContainer square',
+    );
+  });
+
+  testWidgets(
+    'issue 6 (feedback-pruebas): a "Tus emisoras" grid cell shows the shared '
+    'PluriStationArtFallback for a favourite with no favicon',
+    (tester) async {
+      _setLargeSurfaceSize(tester);
+      final favoritos = FakeServicioFavoritos();
+      final estado = EstadoRadio(
+        audio: FakeServicioAudio(),
+        favoritos: favoritos,
+        radio: FakeServicioRadio(),
+        servicioEcualizador: FakeServicioEcualizador(),
+        servicioGrabacion: FakeServicioGrabacionRadio(),
+        resolverArchivoCustom: _archivoCustomVacio,
+        iniciarAutomaticamente: false,
+      );
+      addTearDown(estado.dispose);
+      await tester.runAsync(estado.inicializar);
+      await favoritos.agregar(
+        emisoraDemo(uuid: 'grid-sin-arte', nombre: 'Grid Sin Arte'),
+      );
+      await estado.cargarFavoritos();
+
+      await tester.pumpWidget(
+        _conProviders(estado, _testApp(const PantallaInicio())),
+      );
+      await _pumpStableFrame(tester);
+
+      expect(
+        find.byType(PluriStationArtFallback),
+        findsWidgets,
+        reason:
+            'issue 6: the grid cell must reach the shared fallback, not its '
+            'own flat primaryContainer square',
+      );
+    },
+  );
 }
 
 /// Mirrors the app.dart wiring: EstadoRadio owns the domain notifiers and

test/pantallas/pantalla_reproductor_test.dart

@@ -12,6 +12,7 @@ import 'package:pluriwave/pantallas/pantalla_reproductor.dart';
 import 'package:pluriwave/tema/pluriwave_tokens.dart';
 import 'package:pluriwave/widgets/ecualizador_widget.dart';
 import 'package:pluriwave/widgets/pluri_glass_surface.dart';
+import 'package:pluriwave/widgets/pluri_station_art_fallback.dart';
 import 'package:pluriwave/widgets/visualizador_audio.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -704,4 +705,30 @@ void main() {
       },
     );
   });
+
+  group('issue 6 (feedback-pruebas): shared station-art fallback', () {
+    testWidgets(
+      'a station with no favicon shows the shared PluriStationArtFallback, '
+      'not a flat coloured square',
+      (tester) async {
+        // `emisora` (the file-level fixture) never sets a favicon.
+        final estado = crearEstado();
+        addTearDown(estado.dispose);
+
+        await montarPantalla(tester, estado);
+
+        final arte = find.byKey(const Key('player-hero-art'));
+        expect(
+          find.descendant(
+            of: arte,
+            matching: find.byType(PluriStationArtFallback),
+          ),
+          findsOneWidget,
+          reason:
+              'issue 6: the full player must reach the shared fallback, '
+              'not its own flat primaryContainer square',
+        );
+      },
+    );
+  });
 }