Filter files when selecting a firmware file

When selecting a firmware image, the file selector should only show files that end in .sfw, however it currently shows all files. Tested on Debian 12 using wayland.

The code for this is in the MainWindow::updateFirmwareUi method in client/desktop/mainwindow.cpp.

This issue is actually present in many places where the file selector is used. However, the file chooser shown when selecting File -> Import -> CSV is an example where this works as expected.

This is not a big deal, but an opportunity to optimize.