feat: expose radius to config

[magnus-rattlehead]

Wanted to expose the radius options to config, personally I prefer no rounded corners

[romanstingler]

let's wait for
@MalpenZibo 's opinion

from a technical standpoint, add validation rejecting negative radius values via a custom deserializer like opacity_deserializer.
Add [appearance.radius] to the example config in docs/src/core/config-system.md (pre-existing).

[appearance.radius]
sm = 4.0
md = 8.0
lg = 16.0
xl = 32.0

also @MalpenZibo stylisitc question
Field-level #[serde(default)] could be used instead of struct-level. Struct-level #[serde(default)] is fine and more concise, but the rest of Appearance uses field-level attrs (e.g. pub menu: MenuAppearance relies on MenuAppearance's own Default). Either style is acceptable here since Radius has a Default impl

[MalpenZibo]

Yes, I need to work on this since the last release. I had to decide the right direction. I prefer to work on this stuff personally, so I'm not sure if I will merge this PR. For sure, I will take this into consideration when defining the new option that I plan to add.

#615