Add Default user agent string to Kotatsu

[ghost]

Describe your suggested feature

Many sources have cloudflare protection. Adding a user agent string setting for these would be very helpful for readers.

https://www.whatismybrowser.com/guides/the-latest-user-agent/chrome

Acknowledgements

• I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open issue.

[Koitharu]

There might be some issues with parsers that requires only mobile or desktop useragent (for websites that returns mobile or desktop version depends on useragent header)

[leomonta]

Still.., would it be possible to have a set of usable UAs (like is the case for individual sources) instead of a fully custom one? Mainly to not broadcast my phone ID to all the manga sources in existence.