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/app_ja.arb

@@ -638,6 +638,13 @@
     }
   },
   "snoozeAgainAction": "もう一度スヌーズ",
+  "alarmRingingNotificationTitle": "PluriWave アラーム",
+  "alarmFireChannelName": "鳴っているアラーム",
+  "alarmFireChannelDescription": "音楽アラームが鳴るときの緊急の音と画面",
+  "alarmPreNoticeChannelName": "アラームの事前通知",
+  "alarmPreNoticeChannelDescription": "アラーム前のサイレント通知",
+  "openFolderChooserTitle": "フォルダを開く",
+  "openRecordingChooserTitle": "録音を開く",
   "@preNoticeCountdown": {
     "placeholders": {
       "minutes": {