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/widgets/pluri_wave_scaffold.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../tema/pluriwave_theme.dart';
+import '../tema/pluriwave_tokens.dart';
 
 class PluriWaveScaffold extends StatelessWidget {
   const PluriWaveScaffold({
@@ -31,10 +32,10 @@ class PluriWaveScaffold extends StatelessWidget {
             begin: Alignment.topLeft,
             end: Alignment.bottomRight,
             colors: [
-              const Color(0xFF07121A),
-              const Color(0xFF0D1B24),
-              const Color(0xFF0E4A4F),
-              const Color(0xFF07121A),
+              t.deepViolet,
+              Theme.of(context).colorScheme.surface,
+              PluriWaveTokens.auroraTeal,
+              t.deepViolet,
             ],
             stops: const [0, 0.34, 0.68, 1],
           ),
@@ -45,17 +46,28 @@ class PluriWaveScaffold extends StatelessWidget {
             Positioned(
               left: -120,
               top: -120,
-              child: _AuroraOrb(size: 300, color: const Color(0xFF21D4D9).withValues(alpha: 0.18)),
+              child: _AuroraOrb(
+                size: 300,
+                color: t.electricMagenta.withValues(alpha: 0.18),
+              ),
             ),
             Positioned(
               right: -150,
               top: 160,
-              child: _AuroraOrb(size: 340, color: const Color(0xFF7EE4C2).withValues(alpha: 0.12)),
+              child: _AuroraOrb(
+                size: 340,
+                color: Theme.of(
+                  context,
+                ).colorScheme.secondary.withValues(alpha: 0.12),
+              ),
             ),
             Positioned(
               left: -90,
               bottom: 80,
-              child: _AuroraOrb(size: 260, color: t.warmCoral.withValues(alpha: 0.10)),
+              child: _AuroraOrb(
+                size: 260,
+                color: t.warmCoral.withValues(alpha: 0.10),
+              ),
             ),
             Positioned.fill(
               child: IgnorePointer(