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/l10n/app_es.arb

@@ -568,6 +568,16 @@
   "weekdayShortThursday": "Jue",
   "weekdayShortFriday": "Vie",
   "weekdayShortSaturday": "Sáb",
-  "weekdayShortSunday": "Dom"
-
+  "weekdayShortSunday": "Dom",
+  "stationCount": "{count, plural, =1{1 emisora} other{{count} emisoras}}",
+  "alarmIconLabel": "Alarma musical",
+  "vacationIconLabel": "Modo vacaciones",
+  "streamUrlHint": "https://stream.example.com:8000/radio",
+  "@stationCount": {
+    "placeholders": {
+      "count": {
+        "type": "int"
+      }
+    }
+  }
 }