i18n(alarm): localize all native notification, channel and chooser texts

Centralize every native-side user-facing string in a single
AlarmNotificationStrings store written by Flutter via a new
setNotificationStrings MethodChannel whenever the app locale changes,
and read at notification/channel build time (with English fallbacks)
even when the engine is dead. This replaces the hardcoded Spanish text
in the ringing notification ("Alarma PluriWave", "Posponer", "Detener"),
the pre-notice notification ("Posponer", "Omitir esta vez"), both
notification channels (names + descriptions) and the file-action
choosers ("Abrir carpeta", "Abrir grabación").

The per-alarm preNoticeTemplate/snoozeCountdown template+label args are
dropped from scheduleAlarm and the persisted spec and folded into the
shared store, so a locale change now also relocalizes already-scheduled
alarms. Channels are re-created on each use so their name/description
refresh after a language switch.

Adds alarmRingingNotificationTitle, alarmFire/PreNoticeChannelName,
alarmFire/PreNoticeChannelDescription and openFolder/openRecording
chooser keys across all 13 locales (reusing snoozeAction, stopAlarmAction,
skipNextAction, snoozeAgainAction). Rewrites the template test around
setNotificationStrings. Kotlin is static-reviewed only; no Android build
environment available here.

lib/l10n/gen/app_localizations_es.dart

@@ -1273,6 +1273,29 @@ class AppLocalizationsEs extends AppLocalizations {
   @override
   String get snoozeAgainAction => 'Posponer otra vez';
 
+  @override
+  String get alarmRingingNotificationTitle => 'Alarma PluriWave';
+
+  @override
+  String get alarmFireChannelName => 'Alarmas sonando';
+
+  @override
+  String get alarmFireChannelDescription =>
+      'Sonido y pantalla urgente cuando una alarma musical debe sonar';
+
+  @override
+  String get alarmPreNoticeChannelName => 'Preavisos de alarmas';
+
+  @override
+  String get alarmPreNoticeChannelDescription =>
+      'Notificaciones silenciosas antes de la alarma';
+
+  @override
+  String get openFolderChooserTitle => 'Abrir carpeta';
+
+  @override
+  String get openRecordingChooserTitle => 'Abrir grabación';
+
   @override
   String get eqDeviceEditTitle => 'Editar dispositivo';