feat(app): add onboarding and harden alarms

lib/servicios/servicio_alarmas_android.dart

@@ -74,7 +74,7 @@ class ServicioAlarmasAndroid {
     debugPrint(
       '[PluriWave][alarmas] programar id=${alarma.id} nombre=${alarma.nombre} proxima=${proxima.toIso8601String()} preaviso=${proxima.subtract(const Duration(minutes: 30)).toIso8601String()}',
     );
-    await _channel.invokeMethod<void>('scheduleAlarm', {
+    final programada = await _channel.invokeMethod<bool>('scheduleAlarm', {
       'id': alarma.id,
       'title': alarma.nombre,
       'triggerAtMillis': proxima.millisecondsSinceEpoch,
@@ -85,6 +85,11 @@ class ServicioAlarmasAndroid {
       'fallbackSound': alarma.sonidoInterno.name,
       'volume': alarma.volumen,
     });
+    if (programada != true) {
+      throw StateError(
+        'Android no pudo programar una alarma exacta. Revisa el permiso de alarmas exactas.',
+      );
+    }
   }
 
   Future<void> cancelar(String alarmaId) =>
@@ -96,6 +101,13 @@ class ServicioAlarmasAndroid {
   Future<void> detenerSonidoNativo(String alarmaId) =>
       _logAndInvokeVoid('stopNativeAlarmSound', {'id': alarmaId});
 
+  Future<bool> solicitarPermisoAlarmasExactas() async {
+    final abierto = await _channel.invokeMethod<bool>(
+      'requestExactAlarmPermission',
+    );
+    return abierto ?? false;
+  }
+
   Future<DiagnosticoAlarmasAndroid> diagnostico() async {
     debugPrint('[PluriWave][alarmas] diagnostico android');
     final raw = await _channel.invokeMethod<Map<Object?, Object?>>(