Fix: Active peer notification in background

[callebtc]

Summary

This PR fixes an issue where the "Active Peers" notification (e.g. "bitchatters nearby!") was not triggering when the app was in the background. It also fixes a logic flaw where reconnecting to previously seen peers after a disconnect (0 peers) would not trigger the notification.

Changes

• BluetoothMeshService.kt: Modified onPeerListUpdated to explicitly call serviceNotificationManager.showActiveUserNotification(peerIDs) when the UI delegate is null (indicating the app is in the background). This ensures the notification logic is executed even when the UI is not active.
• NotificationManager.kt: Modified showActiveUserNotification to clear notificationIntervalManager.recentlySeenPeers when the peer list is empty. This resets the tracking state so that subsequent connections (going from 0 to n peers) are treated as "new" connections, allowing the notification to trigger again.

Testing

• Verified that notifications now appear when the app is backgrounded and peers connect.
• Verified that disconnecting and reconnecting triggers the notification again.