Game Files Backup Not Working

**Game information**

This problem affects all games. The game files cannot be backed up.

**Distribution name and version where applicable**

Fedora 39

**Flatpak info**

Flatpak 1.15.6
Steam flatpak 1.0.0.78

**Problem description**

I was about to switch from the Flatpak version to the RPM version to see if my Cloud Save issues would go away. I went to back up the game files so I wouldn't have to download them again. I got an error message when I clicked the Browse button to select a destination folder. I managed to work around it by using flatseal to add the path to the allowed paths. I still got the error message but it worked.

Steps to reproduce:

1. Right click a game in the library
2. Click Properties
3. Click Installed Files in the list on the left
4. Click the 'Backup game files' button on the right
5. Click Browse
6. Notice the error message "The folder contents could not be displayed" and "Operation not supported".
7. Select a folder on the left and navigate to another folder in the main view
8. Click Select on the top right
9. Notice the odd path
10. Click the 'Create backup' button
11. Notice that backing up the game files doesn't do anything

**Does this issue reproduce with native Steam**

Have not tested. Unlikely since it seems to be sandboxing issue.

Start by reproducing the backup flow in the Steam Flatpak on Fedora 39, following the listed steps from a game's Installed Files page through Browse and Create backup. Compare the behavior with the reported Flatpak and Flatseal path permissions; done means selecting a destination works and the game files backup is actually created.

Written by the indexing model from the issue text.