Clarify GRAPE docs and dependency usage

[Codex]

The GRAPE follow-up fixes ensure the new singleflight usage is declared as a direct dependency and the API manual’s error-handling example compiles as shown.

• Dependency: promote golang.org/x/sync to a direct requirement in go.mod to match singleflight usage in the GRAPE client.
• Documentation: update the GRAPE API manual error-handling example to import errors when demonstrating errors.As.

import "errors"

if err := client.RegisterDevice(mac, url); err != nil {
    var apiErr grape.APIError
    if errors.As(err, &apiErr) {
        fmt.Println(apiErr.StatusCode, apiErr.Message)
    }
}