fix(app): hide session metadata from public lock screen

Keep the ongoing VPN notification visible while marking its timer and speed content private so Android does not expose session metadata in public lock-screen previews.

lib/components/notification.dart

@@ -65,7 +65,9 @@ class VpnNotificationManager {
       playSound: false,
       enableVibration: false,
       showWhen: false,
-      visibility: NotificationVisibility.public,
+      // Session time/speed is usage metadata. Keep it out of public lock-screen
+      // previews while leaving the ongoing VPN status notification intact.
+      visibility: NotificationVisibility.private,
       usesChronometer: false,
       category: AndroidNotificationCategory.service,
       colorized: true,