admin UI: add anonymous user creation checkbox (#8773)

Add an "Anonymous" checkbox next to the username field in the Create User
modal. When checked, the username is set to "anonymous" and the credential
generation checkbox is disabled since anonymous users do not need keys.

The checkbox is only shown when no anonymous user exists yet. The
manage-access-keys button in the users table is hidden for the anonymous
user.

weed/admin/dash/admin_data.go

@@ -48,10 +48,11 @@ type ObjectStoreUser struct {
 }
 
 type ObjectStoreUsersData struct {
-	Username    string            `json:"username"`
-	Users       []ObjectStoreUser `json:"users"`
-	TotalUsers  int               `json:"total_users"`
-	LastUpdated time.Time         `json:"last_updated"`
+	Username         string            `json:"username"`
+	Users            []ObjectStoreUser `json:"users"`
+	TotalUsers       int               `json:"total_users"`
+	HasAnonymousUser bool              `json:"has_anonymous_user"`
+	LastUpdated      time.Time         `json:"last_updated"`
 }
 
 // User management request structures