fix(alarm): declare pendingMissedIntent nullable to stop STOP_NATIVE NPE

cancelAutoSilence calls pendingMissedIntent with FLAG_NO_CREATE, the
one Android flag whose entire purpose is to make getBroadcast() return
null on no match. The Kotlin signature declared a non-null return type,
so the compiler-inserted assertion threw NPE inside onStartCommand,
crashing the process before the Dart-side postpone flow could reach
its actual +N-minute reschedule call.

android/app/src/main/kotlin/es/freetimelab/pluriwave/AlarmScheduler.kt

@@ -741,7 +741,10 @@ class AlarmScheduler(private val context: Context) {
             appContext,
             id,
             PendingIntent.FLAG_UPDATE_CURRENT
-        )
+        ) ?: run {
+            Log.w(tag, "alarm.autoSilence arm failed: pendingMissedIntent returned null id=$id")
+            return
+        }
         try {
             alarmManager.setExactAndAllowWhileIdle(
                 AlarmManager.RTC_WAKEUP,

android/app/src/main/kotlin/es/freetimelab/pluriwave/PluriWaveAlarmReceiver.kt

@@ -286,7 +286,7 @@ class PluriWaveAlarmReceiver : BroadcastReceiver() {
         private fun requestCode(id: String, slot: Int): Int = 47 * id.hashCode() + slot
 
         /** Shared PendingIntent factory for the MISSED transition alarm (Decision 3). */
-        fun pendingMissedIntent(context: Context, alarmId: String, flags: Int): PendingIntent =
+        fun pendingMissedIntent(context: Context, alarmId: String, flags: Int): PendingIntent? =
             PendingIntent.getBroadcast(
                 context,
                 requestCode(alarmId, 4),