feat: update agent button on server page

proto/keymanager/v1/keymanager.proto

@@ -25,8 +25,9 @@ message RegisterResponse {
 }
 
 message SyncRequest {
-  string server_id   = 1;
-  string agent_token = 2;
+  string server_id    = 1;
+  string agent_token  = 2;
+  string agent_version = 3;
 }
 
 message SyncResponse {
@@ -69,6 +70,7 @@ message ServerCommand {
   oneof command {
     GenerateKeyCmd generate_key = 2;
     DeleteKeyCmd   delete_key   = 3;
+    UpdateAgentCmd update_agent = 4;
   }
 }
 
@@ -76,6 +78,11 @@ message DeleteKeyCmd {
   string label = 1;
 }
 
+message UpdateAgentCmd {
+  string version       = 1; // e.g. "1.2.3"
+  string gitea_base_url = 2; // e.g. "https://gitea.example.com"
+}
+
 message GenerateKeyCmd {
   string label      = 1;
   string key_type   = 2; // ed25519 | rsa | ecdsa (default: ed25519)