feat: multi-server failover forwarding + reorder/enable

Bark servers now form an ordered failover chain: forwarding tries each
enabled server in list order, stopping at the first success.

- model: BarkServer.enabled (defaults true; legacy JSON compatible)
- core/BarkSender.pushToFirst: ordered try-until-success helper, shared
  by NotifyListenerService / PhoneCallMonitor / RetryWorker
- BarkClient.push uses msg.copy(deviceKey=server.deviceKey) so each
  server pushes its own key
- SettingsStore: drop currentServer/setCurrent; add moveServer +
  setServerEnabled; NotificationProcessor no longer sources deviceKey
- UI: ServerRow RadioButton -> Checkbox + per-row test/move-up/move-down;
  remove top "send test push" button; HomeScreen shows enabled count
- version 1.5.1 -> 1.6.0 (code 9 -> 10); CLAUDE.md synced

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -13,8 +13,8 @@ android {
         applicationId = "com.a2i.forwarder"
         minSdk = 34
         targetSdk = 36
-        versionCode = 9
-        versionName = "1.5.1"
+        versionCode = 10
+        versionName = "1.6.0"
     }
 
     signingConfigs {