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.

lib/widgets/fila_emisora_plana.dart

@@ -8,6 +8,7 @@ import '../l10n/display_names.dart';
 import '../l10n/gen/app_localizations.dart';
 import '../modelos/emisora.dart';
 import '../tema/pluriwave_tokens.dart';
+import 'pluri_station_art_fallback.dart';
 
 /// Item 23 / audit 4.3 + 6.5 (t4:226-232, 302-306): a flat, background-less
 /// station row — square thumbnail, name, meta line, and a caller-supplied
@@ -234,10 +235,10 @@ class _ArteFilaEmisora extends StatelessWidget {
         imageUrl: emisora.favicon!,
         fit: BoxFit.cover,
         placeholder: (_, __) => _shimmer(theme),
-        errorWidget: (_, __, ___) => _iconoFallback(theme),
+        errorWidget: (_, __, ___) => _iconoFallback(),
       );
     }
-    return _iconoFallback(theme);
+    return _iconoFallback();
   }
 
   Widget _shimmer(ThemeData theme) => shimmer.Shimmer.fromColors(
@@ -246,12 +247,9 @@ class _ArteFilaEmisora extends StatelessWidget {
     child: Container(color: theme.colorScheme.surfaceContainerHighest),
   );
 
-  Widget _iconoFallback(ThemeData theme) => Container(
-    color: theme.colorScheme.primaryContainer,
-    child: Icon(
-      Icons.radio_rounded,
-      size: 22,
-      color: theme.colorScheme.onPrimaryContainer,
-    ),
-  );
+  // Issue 6 (feedback-pruebas): was a flat `primaryContainer` square with a
+  // bare `radio_rounded` icon — now the same shared fallback every other
+  // surface uses.
+  Widget _iconoFallback() =>
+      PluriStationArtFallback(seed: emisora.uuid, iconSize: 22);
 }