feat(ui): design token discipline, accessibility and i18n pass

- Replace all hardcoded Color literals outside lib/tema with theme tokens (new static brand palette in PluriWaveTokens); media notification uses the brand color instead of the Material default purple
- Favorite button on station cards grows to a 48dp target and becomes an independent semantics node for screen readers (Semantics container fix)
- All flutter_animate call sites route through the PluriAnimate reduced-motion gate (zero direct .animate() left)
- Locale-aware short dates via intl DateFormat (new lib/l10n/formato_fechas.dart) replacing the hardcoded DD/MM/YYYY; proper plural messages for the favorites counter; example stream URL as a localized key - all 13 locales
- Rounded shimmer placeholders matching card radii; shimmer loading state in search instead of a bare spinner; rounded icon variants unified in settings; bottom-sheet conventions on the custom station form
- Fix latent debug crash: vacation editor read AppLocalizations in initState
- 11 new tests (121 total green), flutter analyze clean

test/pantallas/pantalla_alarmas_fecha_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:intl/date_symbol_data_local.dart';
+import 'package:intl/intl.dart';
+import 'package:pluriwave/l10n/formato_fechas.dart';
+
+/// S5-R4: short dates must follow the active locale, not a hardcoded
+/// DD/MM/YYYY pattern.
+void main() {
+  test('en-US usa el orden mes/día, no el formato fijo DD/MM/YYYY', () {
+    final fecha = DateTime(2026, 6, 11);
+    final resultado = fechaCortaLocalizada('en-US', fecha);
+
+    expect(resultado, DateFormat.yMd('en-US').format(fecha));
+    expect(resultado, isNot('11/06/2026'));
+    expect(resultado, '6/11/2026');
+  });
+
+  test('es usa el orden día/mes', () async {
+    await initializeDateFormatting('es');
+    final fecha = DateTime(2026, 6, 11);
+
+    expect(
+      fechaCortaLocalizada('es', fecha),
+      DateFormat.yMd('es').format(fecha),
+    );
+  });
+}

test/pantallas/pantalla_buscar_shimmer_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/estado/estado_busqueda.dart';
+import 'package:pluriwave/l10n/gen/app_localizations.dart';
+import 'package:pluriwave/pantallas/pantalla_buscar.dart';
+import 'package:pluriwave/widgets/tarjeta_emisora.dart';
+import 'package:provider/provider.dart';
+
+import '../helpers/fakes.dart';
+
+/// S5-R6: the search loading state uses shimmer placeholders, not a bare
+/// spinner, to stay consistent with the rest of the app.
+class _BusquedaCargando extends EstadoBusqueda {
+  _BusquedaCargando() : super(radio: FakeServicioRadio());
+
+  @override
+  bool get cargando => true;
+}
+
+void main() {
+  testWidgets('PantallaBuscar muestra shimmer mientras carga', (tester) async {
+    final busqueda = _BusquedaCargando();
+    addTearDown(busqueda.dispose);
+
+    await tester.pumpWidget(
+      ListenableProvider<EstadoBusqueda>.value(
+        value: busqueda,
+        child: MaterialApp(
+          locale: const Locale('es'),
+          localizationsDelegates: AppLocalizations.localizationsDelegates,
+          supportedLocales: AppLocalizations.supportedLocales,
+          home: const Scaffold(body: PantallaBuscar()),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    expect(find.byType(TarjetaEmisoraShimmer), findsWidgets);
+    expect(find.byType(CircularProgressIndicator), findsNothing);
+  });
+}

test/pantallas/pantalla_favoritos_plural_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/l10n/gen/app_localizations.dart';
+
+/// S5-R5: bare counters must use proper ARB plural messages.
+void main() {
+  test('stationCount cambia entre singular y plural (en)', () async {
+    final l10n = await AppLocalizations.delegate.load(const Locale('en'));
+
+    expect(l10n.stationCount(1), '1 station');
+    expect(l10n.stationCount(5), '5 stations');
+  });
+
+  test('stationCount cambia entre singular y plural (es)', () async {
+    final l10n = await AppLocalizations.delegate.load(const Locale('es'));
+
+    expect(l10n.stationCount(1), isNot(l10n.stationCount(5)));
+    expect(l10n.stationCount(5), contains('5'));
+  });
+}