fix(alarm): localize pre-notice countdown and fix snooze dismiss

Replace hardcoded Spanish pre-notice text with computed remaining
minutes using l10n template passed via MethodChannel. Fix snooze
dismiss in dead-app state with canPop guard and SystemNavigator.pop
fallback.

lib/pantallas/pantalla_alarma_sonando.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:just_audio/just_audio.dart';
 import 'package:provider/provider.dart';
 
@@ -153,11 +154,10 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
   Future<void> _detener() async {
     final radio = context.read<EstadoRadio>();
     final alarmas = context.read<EstadoAlarmas>();
-    final navigator = Navigator.of(context);
     await _liberarAudioLocal();
     await radio.audio.pausar();
     await alarmas.finalizarEjecucion(widget.alarma.id);
-    if (mounted) navigator.pop();
+    if (mounted) _dismissScreen();
   }
 
   /// Flutter-first snooze (S2-R1): tears down local audio, then routes
@@ -166,11 +166,25 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
   Future<void> _posponer(int minutos) async {
     final radio = context.read<EstadoRadio>();
     final alarmas = context.read<EstadoAlarmas>();
-    final navigator = Navigator.of(context);
     await _liberarAudioLocal();
     await radio.audio.pausar();
     await alarmas.posponerAlarma(widget.alarma, minutos);
-    if (mounted) navigator.pop();
+    if (mounted) _dismissScreen();
+  }
+
+  /// Dismisses the alarm screen safely in both live-app and dead-app states.
+  ///
+  /// When the alarm screen is the root activity (launched via full-screen intent
+  /// from a dead app), [Navigator.canPop] returns false and calling
+  /// [Navigator.pop] would be a no-op. In that case [SystemNavigator.pop] is
+  /// used to call `Activity.finish()` and return to the home screen.
+  void _dismissScreen() {
+    final navigator = Navigator.of(context);
+    if (navigator.canPop()) {
+      navigator.pop();
+    } else {
+      SystemNavigator.pop();
+    }
   }
 
   List<int> _opcionesSnooze() {