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

lib/tema/pluri_animate.dart

@@ -33,6 +33,20 @@ extension PluriAnimate on Widget {
         .scaleXY(begin: begin, end: 1, duration: duration, curve: curve);
   }
 
+  /// Fade + subtle vertical slide entry animation.
+  Widget pluriFadeSlideIn(
+    BuildContext context, {
+    Duration duration = const Duration(milliseconds: 350),
+    Duration delay = Duration.zero,
+    Curve curve = Curves.easeOutCubic,
+    double beginY = 0.1,
+  }) {
+    if (_animacionesDeshabilitadas(context)) return this;
+    return animate(delay: delay)
+        .fadeIn(duration: duration, curve: curve)
+        .slideY(begin: beginY, end: 0, duration: duration, curve: curve);
+  }
+
   bool _animacionesDeshabilitadas(BuildContext context) =>
       MediaQuery.maybeDisableAnimationsOf(context) ?? false;
 }