feat(tokens): add design tokens, type scale, push scaffold, and root nav state

lib/app.dart

@@ -8,6 +8,7 @@ import 'estado/estado_grabacion.dart';
 import 'estado/estado_radio.dart';
 import 'estado/estado_alarmas.dart';
 import 'estado/estado_idioma.dart';
+import 'estado/estado_navegacion.dart';
 import 'l10n/display_names.dart';
 import 'l10n/gen/app_localizations.dart';
 import 'modelos/alarma_musical.dart';
@@ -70,6 +71,9 @@ class PluriWaveApp extends StatelessWidget {
         ChangeNotifierProvider(
           create: (_) => EstadoIdioma(sharedPreferences: prefs),
         ),
+        // Design ADR-8: root-to-root navigation state. `_PaginaPrincipal`
+        // watches this instead of owning `_indice` locally.
+        ChangeNotifierProvider(create: (_) => EstadoNavegacionRaiz()),
       ],
       child: Consumer<EstadoIdioma>(
         builder:
@@ -98,7 +102,6 @@ class _PaginaPrincipal extends StatefulWidget {
 
 class _PaginaPrincipalState extends State<_PaginaPrincipal>
     with WidgetsBindingObserver {
-  int _indice = 0;
   StreamSubscription<String>? _errorSubscription;
   StreamSubscription<EventoAlarmaAndroid>? _alarmaSubscription;
   StreamSubscription<AlarmaMusical>? _alarmaVencidaSubscription;
@@ -206,6 +209,8 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
+    final navegacion = context.watch<EstadoNavegacionRaiz>();
+    final indice = navegacion.indice;
 
     return PluriWaveScaffold(
       appBar: AppBar(
@@ -236,8 +241,8 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
                 ),
               ),
           child: KeyedSubtree(
-            key: ValueKey<int>(_indice),
-            child: _paginas[_indice],
+            key: ValueKey<int>(indice),
+            child: _paginas[indice],
           ),
         ),
       ),
@@ -252,8 +257,8 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
               const MiniReproductor(),
               PluriBottomNavigation(
                 items: _navItems(l10n),
-                selectedIndex: _indice,
-                onSelected: (i) => setState(() => _indice = i),
+                selectedIndex: indice,
+                onSelected: (i) => navegacion.irA(RaizPluriWave.values[i]),
               ),
             ],
           ),
@@ -307,7 +312,7 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
     if (evento.accion.endsWith('.SKIP_NEXT')) {
       await estado.saltarProxima(alarma.id);
       if (!mounted) return;
-      setState(() => _indice = 3);
+      context.read<EstadoNavegacionRaiz>().irA(RaizPluriWave.alarmas);
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(
           content: Text(
@@ -330,7 +335,7 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
         ejecucion,
       );
       if (!mounted) return;
-      setState(() => _indice = 3);
+      context.read<EstadoNavegacionRaiz>().irA(RaizPluriWave.alarmas);
       // posponerProximaDesdePreaviso no longer throws on a native scheduling
       // failure — it records the failure into EstadoAlarmas.error instead.
       // Branch on it here so the user sees the real outcome instead of an
@@ -347,7 +352,7 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
       return;
     }
     if (evento.accion.endsWith('.PRE_NOTICE')) {
-      setState(() => _indice = 3);
+      context.read<EstadoNavegacionRaiz>().irA(RaizPluriWave.alarmas);
       return;
     }
     await _mostrarAlarmaSonando(alarma);