enhance(ui): use OpenAPI 3.0 spec in API viewer, add spec buttons (#38478)

Follow-ups from https://github.com/go-gitea/gitea/pull/37038:

- Render the OpenAPI 3.0 spec in the API viewer, it is richer than the
Swagger 2.0 rendering
- Replace the back link with gitea-styled buttons to view both specs and
return to Gitea, flowing above swagger-ui on viewports where they would
overlap the title
- Key `VisibilityModes` by the string enum type, now named
`VisibilityString`, removing the `string()` casts at API call sites
- Drop the raw visibility strings from the `Service` settings struct in
favor of the typed mode fields, which also removes the org default
visibility row from the admin config page
- Fix two pre-existing issues surfaced in review: an invalid
`DEFAULT_USER_VISIBILITY` was silently accepted as public, and the org
visibility error message showed the enum zero value instead of the
submitted input

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/convert/user.go

@@ -65,7 +65,7 @@ func toUser(ctx context.Context, user *user_model.User, signed, authed bool) *ap
 		StarredRepos: user.NumStars,
 	}
 
-	result.Visibility = api.UserVisibility(user.Visibility.String())
+	result.Visibility = api.VisibilityString(user.Visibility.String())
 
 	// hide primary email if API caller is anonymous or user keep email private
 	if signed && (!user.KeepEmailPrivate || authed) {