fix(server): build ping address with net.JoinHostPort for IPv6 hosts

fmt.Sprintf("%s:%d", host, port) produces a malformed address for IPv6
literal hosts (go vet: "address format does not work with IPv6").

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/server/handlers.go

@@ -1695,7 +1695,7 @@ func (s *Server) handleCollectStart(w http.ResponseWriter, r *http.Request) {
 // pingHost dials host:port up to total times with 2s timeout each, returns true if
 // at least need attempts succeeded.
 func pingHost(host string, port int, total, need int) (bool, string) {
-	addr := fmt.Sprintf("%s:%d", host, port)
+	addr := net.JoinHostPort(host, strconv.Itoa(port))
 	var successes atomic.Int32
 	done := make(chan struct{}, total)
 	for i := 0; i < total; i++ {