feat(alarms): native reliability fixes and end-to-end snooze

- Use mediaPlayback|systemExempted FGS type with FOREGROUND_SERVICE_SYSTEM_EXEMPTED so alarms fire on Android 14+ (FOREGROUND_SERVICE_ALARM does not exist in the SDK)
- Deduplicate fire notifications: the foreground service FSI notification is the single owner; receiver path removed
- Notification channel v2 with alarm sound URI and USAGE_ALARM attributes, one-time guarded migration from legacy channels
- Pass fallback station through the MethodChannel (NativeAlarmSpec schemaVersion 3) with a three-stage audio chain: primary -> fallback station -> bundled WAV
- Native fade-in volume ramp honoring fadeInSegundos when the app is killed
- Request battery-optimization exemption once, tracked with a persisted asked-once flag
- Fix snooze end-to-end: native ACTION_SNOOZE now reports back to Flutter (snoozed event + cold-start sync), snooze anchor unified to occurrence+minutes on both sides, periodic recalc no longer erases an active snooze
- Add snooze buttons (3/5/10/custom) to the ringing screen with shared audio teardown
- Redesign ringing screen on PluriWaveScaffold with reduced-motion-aware entry animation (new PluriAnimate helper)
- Alarm editor: live next-trigger preview, searchable station pickers (primary and fallback), configurable snooze duration, volume floor down to 0
- New alarm strings localized across all 13 locales
- New unit/widget tests for the snooze flow, alarm bridge payloads, ringing screen and editor (77 tests green)
- SDD artifacts for the app-quality-and-native-alarms change (explore, proposal, spec, design, tasks, apply progress)

lib/l10n/app_fr.arb

@@ -422,6 +422,18 @@
   "soundDigitalPulse": "Impulsion numérique",
   "favoriteStationLabel": "Station favorite",
   "noStationUseInternalSound": "Aucune station : utiliser le son interne",
+  "alarmFallbackStationLabel": "Station de secours",
+  "alarmStationPickerSearchHint": "Rechercher une station par nom",
+  "alarmSnoozeDurationTitle": "Durée de répétition",
+  "snoozeAction": "Répéter",
+  "alarmSnoozeOptionLabel": "{minutes} min",
+  "@alarmSnoozeOptionLabel": {
+    "placeholders": {
+      "minutes": {
+        "type": "int"
+      }
+    }
+  },
   "saveFavoritesAlarmHint": "Enregistrez des stations dans les Favoris pour les utiliser comme alarme musicale.",
   "useCurrentStationAction": "Utiliser la station actuelle",
   "playDuringVacations": "Sonner pendant les vacances",