change user-agent to default user agent of presumably the webview of the device.

[NagoorBabuSir]

As suggested in https://github.com/Adamantcheese/Kuroba/issues/1431#issuecomment-1654948439 by cheese himself.

It's reported that this still doesn't work perfectly, most notably a plethora of users say they cannot post images (until a number of text-only posts are made), but it's better than not being able to post at all.

[Adamantcheese]

Closed with 003fe47c0f0547c497aac5d3b10f2e30a2b28e30.