fix(alarm): handle snooze reschedule failures instead of silently dropping them

posponerAlarma() and posponerProximaDesdePreaviso() called the native
scheduler with no error handling, unlike guardarAlarma(). When the
native call failed (e.g. revoked exact-alarm permission), the
exception escaped before notifyListeners() ran, leaving the alarm
list stuck on stale data with no real alarm scheduled and no snooze
countdown notification.

Both methods now mirror guardarAlarma()'s pattern: permission
pre-check, try/catch into _error, and an unconditional
notifyListeners() so the UI always reflects the outcome. Failures
surface via SnackBar in the ringing screen and in app.dart's
postpone-next handler.

lib/app.dart

@@ -300,10 +300,16 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal> {
       );
       if (!mounted) return;
       setState(() => _indice = 3);
+      // 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
+      // always-success message.
+      final error = estado.error;
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(
           content: Text(
-            AppLocalizations.of(context).alarmPostponedCurrentExecution,
+            error ??
+                AppLocalizations.of(context).alarmPostponedCurrentExecution,
           ),
         ),
       );