Android app will not work with mtls / client certificates

### Which version of floccus are you using?

Android 5.9.1

### How many bookmarks do you have, roughly?

100

### Are you using other means to sync bookmarks in parallel to floccus?

no

### Sync method

Nextcloud Bookmarks

### Which browser are you using? In case you are using the phone App, specify the Android or iOS version and device please.

_No response_

### Which version of Nextcloud Bookmarks are you using? (if relevant)

_No response_

### Which version of Nextcloud? (if relevant)

_No response_

### What kind of WebDAV server are you using? (if relevant)

_No response_

### Describe the Bug

Domains requiring mtls / client certificates will not work on the Android app. There is a "Nextcloud login failed" error displayed.

### Expected Behavior

The Android system client certificate selector should pop up OR there should be a settings screen where the client certificate could be selected. Note that in the browser version the login works after going to Options and enabling "send client credentials".

### To Reproduce

- protect a domain with mtls
- try to log in via floccus android app to said domain via nextcloud

### Debug log provided

- [ ] I have provided a debug log file