Files silently fail to save, due to the .desktop file Exec setup

### Saving a file silently fails

#### OS INFO
Flatpak version: 1.11.6
OS: Fedora 44 KDE

#### How to reproduce
Open a file. Edit it, click save. Nothing happens. Closing the app -> asks to save before close and does not save.

#### Proposed fix
To fix this I changed the `Exec` line in the `.desktop` file to
```desktop
Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 com.xnview.XnViewMP %f
```

`--command=env` and `QT_QPA_PLATFORMTHEME=gtk3` and the `@` symbols break file save handling when running on KDE it seems. It should still remain working for users who run other desktop environments.

Locate the application's .desktop file and inspect its Exec entry, especially the --command=env, QT_QPA_PLATFORMTHEME=gtk3, and @ arguments. Reproduce the save failure on Fedora KDE, then verify that the corrected launcher saves files while remaining functional in other desktop environments.

Written by the indexing model from the issue text.