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>

tests/integration/api_user_search_test.go

@@ -47,7 +47,7 @@ func TestAPIUserSearchLoggedIn(t *testing.T) {
 	for _, user := range results.Data {
 		assert.Contains(t, user.UserName, query)
 		assert.NotEmpty(t, user.Email)
-		assert.Equal(t, api.UserVisibilityPublic, user.Visibility)
+		assert.Equal(t, api.VisibilityStringPublic, user.Visibility)
 	}
 }
 
@@ -103,7 +103,7 @@ func TestAPIUserSearchAdminLoggedInUserHidden(t *testing.T) {
 	for _, user := range results.Data {
 		assert.Contains(t, user.UserName, query)
 		assert.NotEmpty(t, user.Email)
-		assert.Equal(t, api.UserVisibilityPrivate, user.Visibility)
+		assert.Equal(t, api.VisibilityStringPrivate, user.Visibility)
 	}
 }