[MM-29049] Fix remaining errcheck errors in app, api4 and web package (#31307)

Co-authored-by: Claude <noreply@anthropic.com>

server/channels/api4/config_local.go

@@ -208,5 +208,8 @@ func localGetClientConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	auditRec.Success()
 
-	w.Write([]byte(model.MapToJSON(c.App.Srv().Platform().ClientConfigWithComputed())))
+	_, err := w.Write([]byte(model.MapToJSON(c.App.Srv().Platform().ClientConfigWithComputed())))
+	if err != nil {
+		c.Logger.Warn("Error while writing response", mlog.Err(err))
+	}
 }