fix: register PeerCenterRpc in management API server so CLI peer-center works (#1929)

PeerCenterRpc was only registered in the per-instance peer-to-peer RPC
manager (domain = network_name), but not in the management API server
(domain = ""). The CLI connects to the management API with an empty
domain, causing "Invalid service name: PeerCenterRpc" errors.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

easytier/src/rpc_service/mod.rs

@@ -3,6 +3,7 @@ mod api;
 mod config;
 mod connector_manage;
 mod mapped_listener_manage;
+mod peer_center;
 mod peer_manage;
 mod port_forward_manage;
 mod proxy;
@@ -67,6 +68,14 @@ pub trait InstanceRpcService: Sync + Send {
     ) -> &dyn crate::proto::api::config::ConfigRpc<
         Controller = crate::proto::rpc_types::controller::BaseController,
     >;
+    fn get_peer_center_service(
+        &self,
+    ) -> std::sync::Arc<
+        dyn crate::proto::peer_rpc::PeerCenterRpc<
+                Controller = crate::proto::rpc_types::controller::BaseController,
+            > + Send
+            + Sync,
+    >;
 }
 
 fn get_instance_service(