Use DownloadHandler for setting group icon in GroupDataGrid

[Flaurite]

Description

DownloadHandler.forClassResource() - uses VAADIN/.. URL path, which is configured by Vaadin security by default.

QA

Check that icon is displaying in group column..

[SergeiAksenov2]

Tested on:
Jmix version: 2.7.999-SNAPSHOT
Jmix Studio Plugin Version: 2.8.SNAPSHOT8125-252
IntelliJ version: IntelliJ IDEA 2025.2.6.1