Unable to mount from external or network drive

## Description
If the vault is not located in the user home directory, it cannot be chosen in the file picker dialog started by Cryptomator. Additionally, already existing vaults (i.e., added over the appimage) are not found, if not located in the home dir.

## Reason
Cryptomator flatpak is restricted to [only access the user home directory](https://github.com/flathub/org.cryptomator.Cryptomator/blob/1ec15d8651ebf9a9dd6ed074c2fd584c903d4fa2/org.cryptomator.Cryptomator.yaml#L12).

## Solution
Ensure Cryptomator is not running and then override the option with one of the following:
You can give the Cryptomator Flatpak complete access to your host file system, with the following command:
```
sudo flatpak override org.cryptomator.Cryptomator --filesystem=host
```

You can also only state where on the file system Cryptomator Flatpak should have access (e.g. to a specific directoy):
```
sudo flatpak override org.cryptomator.Cryptomator --filesystem=/media/my_thumb_drive
```

You can provide multiple filesystem options, so you could do any number of mount points, for example:
```
sudo flatpak override org.cryptomator.Cryptomator --filesystem=/media/festplatte --filesystem=home
```

If you prefer to use a GUI tool, you can use [Flatseal](https://flathub.org/apps/details/com.github.tchx84.Flatseal) to modify the permission accordingly.

Taken from https://community.cryptomator.org/t/cryptomator-flatpak-doesnt-find-safes/9613/2?u=infeo

Start with the Flatpak manifest YAML linked in the issue and inspect its filesystem permissions. Verify the configured permissions against the reported external and network-drive cases, then confirm that the Cryptomator file picker and existing vault discovery work outside the home directory.

Written by the indexing model from the issue text.