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.dart

@@ -2324,6 +2324,48 @@ abstract class AppLocalizations {
   /// **'Posponer otra vez'**
   String get snoozeAgainAction;
 
+  /// No description provided for @alarmRingingNotificationTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Alarma PluriWave'**
+  String get alarmRingingNotificationTitle;
+
+  /// No description provided for @alarmFireChannelName.
+  ///
+  /// In es, this message translates to:
+  /// **'Alarmas sonando'**
+  String get alarmFireChannelName;
+
+  /// No description provided for @alarmFireChannelDescription.
+  ///
+  /// In es, this message translates to:
+  /// **'Sonido y pantalla urgente cuando una alarma musical debe sonar'**
+  String get alarmFireChannelDescription;
+
+  /// No description provided for @alarmPreNoticeChannelName.
+  ///
+  /// In es, this message translates to:
+  /// **'Preavisos de alarmas'**
+  String get alarmPreNoticeChannelName;
+
+  /// No description provided for @alarmPreNoticeChannelDescription.
+  ///
+  /// In es, this message translates to:
+  /// **'Notificaciones silenciosas antes de la alarma'**
+  String get alarmPreNoticeChannelDescription;
+
+  /// No description provided for @openFolderChooserTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Abrir carpeta'**
+  String get openFolderChooserTitle;
+
+  /// No description provided for @openRecordingChooserTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Abrir grabación'**
+  String get openRecordingChooserTitle;
+
   /// No description provided for @eqDeviceEditTitle.
   ///
   /// In es, this message translates to: