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_favoritos_plural_test.dart

@@ -0,0 +1,20 @@
+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'));
+  });
+}