Changes:
- Constants.kt: Add KEY_LAST_GLOBAL_SYNC_TIME and MIN_GLOBAL_SYNC_INTERVAL_MS (30s)
- SyncStateManager.kt: Add canSyncGlobally() and markGlobalSyncStarted() methods
- MainViewModel.triggerAutoSync(): Check global cooldown before individual throttle
- MainViewModel.triggerAutoSync(): Mark global sync start before viewModelScope.launch
- MainViewModel.triggerManualSync(): Mark global sync start after tryStartSync()
- SyncWorker.doWork(): Add SyncStateManager.tryStartSync() coordination with silent=true
- SyncWorker.doWork(): Check global cooldown before expensive server checks
- SyncWorker.doWork(): Call markCompleted()/markError() to update SyncStateManager
- WifiSyncReceiver: Add global cooldown check and KEY_SYNC_TRIGGER_WIFI_CONNECT validation
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