Improve user-agent handling

[M-rcus]

Currently the user agent is retrieved using the navigator.userAgent global variable.
This is what the browser reports by default.

However, some power users might be using user agent switchers or Firefox's resistFingerprinting setting, making navigator.userAgent the "wrong" value.

We should probably make use of a 3rd party API to retrieve the user agent that the browser sends to websites, if possible via the extension.

[M-rcus]

The plan, as of right now, is to utilize Cloudflare Workers with my own custom worker: https://github.com/M-rcus/CloudflareWorkers/tree/main/user-agent

I assume that some people, for privacy reasons, might dislike the usage of Cloudflare Workers for user agent detection. So if I'm not really lazy, I'll probably release this feature in two ways:

• v1.0.2 v1.0.3 - Force-enabled for all users, but there are otherwise no changes with the release. It will, however, save a setting that will be used in the future releases (v1.1.0+)
• v1.1.0 (probably) - Opt-in setting for the user agent handling, so people that can't rely on navigator.userAgent can explicitly enable it themselves.

The (sub)domain I plan on using is ua.rcus.dev