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/gen/app_localizations_zh.dart

@@ -773,6 +773,23 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get noStationUseInternalSound => '无电台：使用内部声音';
 
+  @override
+  String get alarmFallbackStationLabel => '备用电台';
+
+  @override
+  String get alarmStationPickerSearchHint => '按名称搜索电台';
+
+  @override
+  String get alarmSnoozeDurationTitle => '贪睡时长';
+
+  @override
+  String get snoozeAction => '贪睡';
+
+  @override
+  String alarmSnoozeOptionLabel(int minutes) {
+    return '$minutes 分钟';
+  }
+
   @override
   String get saveFavoritesAlarmHint => '将电台保存到收藏，即可把它们用作音乐闹钟。';