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>

modules/structs/admin_user.go

@@ -29,7 +29,7 @@ type CreateUserOption struct {
 	// Whether the user has restricted access privileges
 	Restricted *bool `json:"restricted"`
 	// User visibility level: public, limited, or private
-	Visibility UserVisibility `json:"visibility" binding:"In(,public,limited,private)"`
+	Visibility VisibilityString `json:"visibility" binding:"In(,public,limited,private)"`
 
 	// For explicitly setting the user creation timestamp. Useful when users are
 	// migrated from other systems. When omitted, the user's creation timestamp
@@ -78,5 +78,5 @@ type EditUserOption struct {
 	// Whether the user has restricted access privileges
 	Restricted *bool `json:"restricted"`
 	// User visibility level: public, limited, or private
-	Visibility UserVisibility `json:"visibility" binding:"In(,public,limited,private)"`
+	Visibility VisibilityString `json:"visibility" binding:"In(,public,limited,private)"`
 }