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/tarjeta_emisora_thumbnail_test.dart

@@ -2,6 +2,7 @@ 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/pluri_station_art_fallback.dart';
 import 'package:pluriwave/widgets/tarjeta_emisora.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -73,6 +74,24 @@ void main() {
     expect(tester.getSize(thumbnailClip), const Size(48, 48));
   });
 
+  testWidgets(
+    'issue 6 (feedback-pruebas): a station with no favicon renders via the '
+    'shared PluriStationArtFallback (extracted from this exact fallback)',
+    (tester) async {
+      await tester.pumpWidget(
+        host(
+          TarjetaEmisora(
+            emisora: emisoraDemo(uuid: 'sin-arte', nombre: 'Sin Arte'),
+            esCompacta: true,
+          ),
+        ),
+      );
+      await tester.pump();
+
+      expect(find.byType(PluriStationArtFallback), findsOneWidget);
+    },
+  );
+
   testWidgets('esCompacta shimmer placeholder is a square block, not a '
       'circle', (tester) async {
     await tester.pumpWidget(