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/tarjeta_emisora.dart

@@ -10,6 +10,7 @@ import '../modelos/emisora.dart';
 import '../tema/pluriwave_theme.dart';
 import 'pluri_glass_surface.dart';
 import 'pluri_icon.dart';
+import 'pluri_station_art_fallback.dart';
 
 /// Tarjeta compacta para mostrar una emisora en listas y grids.
 /// Incluye botón de favorito visible en ambos modos.
@@ -302,51 +303,12 @@ class _TarjetaEmisoraState extends State<TarjetaEmisora> {
     );
   }
 
+  // Issue 6 (feedback-pruebas): extracted into the shared
+  // `PluriStationArtFallback` so every surface that can render a station
+  // without artwork shows the SAME fallback — see that widget's own doc
+  // comment for why the selection formula must never drift.
   Widget _iconoFallback(double size) {
-    final art = _fallbackArtFor(widget.emisora.uuid);
-    return Stack(
-      fit: StackFit.expand,
-      children: [
-        Image.asset(
-          art,
-          fit: BoxFit.cover,
-          errorBuilder:
-              (_, __, ___) => DecoratedBox(
-                decoration: BoxDecoration(
-                  gradient: LinearGradient(
-                    begin: Alignment.topLeft,
-                    end: Alignment.bottomRight,
-                    colors: [
-                      context.pluriTokens.deepViolet,
-                      context.pluriTokens.electricMagenta.withValues(
-                        alpha: 0.8,
-                      ),
-                    ],
-                  ),
-                ),
-              ),
-        ),
-        Center(
-          child: PluriIcon(
-            glyph: PluriIconGlyph.player,
-            variant: PluriIconVariant.activeGlow,
-            size: size,
-            semanticLabel: AppLocalizations.of(context).stationIconLabel,
-          ),
-        ),
-      ],
-    );
-  }
-
-  String _fallbackArtFor(String seed) {
-    const arts = [
-      'assets/images/station_art_aurora.png',
-      'assets/images/station_art_cosmic.png',
-      'assets/images/station_art_pulse.png',
-      'assets/images/station_art_nova.png',
-    ];
-    final index = seed.codeUnits.fold<int>(0, (a, b) => a + b) % arts.length;
-    return arts[index];
+    return PluriStationArtFallback(seed: widget.emisora.uuid, iconSize: size);
   }
 }