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/widgets/fila_emisora_plana_test.dart

@@ -4,6 +4,7 @@ import 'package:pluriwave/estado/estado_radio.dart';
 import 'package:pluriwave/l10n/gen/app_localizations.dart';
 import 'package:pluriwave/tema/pluriwave_tokens.dart';
 import 'package:pluriwave/widgets/fila_emisora_plana.dart';
+import 'package:pluriwave/widgets/pluri_station_art_fallback.dart';
 import 'package:pluriwave/widgets/tarjeta_emisora.dart';
 import 'package:provider/provider.dart';
 
@@ -77,6 +78,32 @@ void main() {
     );
   });
 
+  testWidgets(
+    'issue 6 (feedback-pruebas): a station with no favicon shows the shared '
+    'PluriStationArtFallback, not a flat coloured square',
+    (tester) async {
+      final estado = _estado();
+      addTearDown(estado.dispose);
+      final emisora = emisoraDemo(uuid: 'sin-arte', nombre: 'Sin Arte');
+
+      await tester.pumpWidget(
+        _host(estado, FilaEmisoraPlana(emisora: emisora, meta: '')),
+      );
+      await tester.pump();
+
+      expect(
+        find.descendant(
+          of: find.byKey(const ValueKey('fila-emisora-plana-arte')),
+          matching: find.byType(PluriStationArtFallback),
+        ),
+        findsOneWidget,
+        reason:
+            'issue 6: the flat row must reach the shared fallback, not its '
+            'own flat primaryContainer square',
+      );
+    },
+  );
+
   testWidgets('omits the meta line entirely when empty (no stray gap)', (
     tester,
   ) async {