fix superfluous call to WriteHeader

the first call to http.ResponseWriter.Write implicitly calls WriteHeader
with http.StatusOK if it hasn't already been called. once WriteHeader
has been called, subsequent calls has no effect. Write is called when
JSON encoding progressUpdateJSON{}. calls to
http.ResponseWriter.WriteHeader after the first encode is useless and
produces a warning:

http: superfluous response.WriteHeader call from github.com/ollama/ollama/server/internal/registry.(*statusCodeRecorder).WriteHeader (server.go:77)

server/internal/registry/server.go

@@ -73,8 +73,13 @@ type statusCodeRecorder struct {
 func (r *statusCodeRecorder) WriteHeader(status int) {
 	if r._status == 0 {
 		r._status = status
-	}
 		r.ResponseWriter.WriteHeader(status)
+	}
+}
+
+func (r *statusCodeRecorder) Write(b []byte) (int, error) {
+	r._status = r.status()
+	return r.ResponseWriter.Write(b)
 }
 
 var (