fix: handle NostrVpn provider in VPN disconnect match

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/vpn.rs

@@ -190,6 +190,12 @@ impl RpcHandler {
                     .output()
                     .await;
             }
+            vpn::VpnProvider::NostrVpn => {
+                let _ = tokio::process::Command::new("systemctl")
+                    .args(["stop", "nostr-vpn"])
+                    .output()
+                    .await;
+            }
         }
 
         info!("VPN disconnected");