Check if auto-upload directory is writable

[cweiske]

Is your feature request related to a problem? Please describe.
I configured Nextcloud on Android to automatically upload photos to the server, into the "SofortUpload" directory. This directory was shared by another user, and I had no write access to it.
The Nextcloud app still tried to upload images again and again to this folder, giving the error notification (German):

Hochladen fehlgeschlagen
Ihnen fehlt die Erlaubnis diesen Ordner hochzuladen

(translation: "Upload failed / You are missing the permission to upload this folder")

Describe the solution you'd like
When selecting a folder in the auto upload configuration, the NextCloud app should check if I have write permissions to it.

Describe alternatives you've considered
Do not show folders I don't have write access to.

Additional context
Nextcloud app on my Android 14 phone spent 11.5 GiB of mobile data in one month trying to upload 150 photos, failing again and again.

Nextcloud android app 3.30.8

[joshtrichards]

Likely similar handling needed for #11372