Check if auto-upload directory is writable

**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