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_test.go

@@ -29,11 +29,11 @@ func TestUser_ToUser(t *testing.T) {
 
 	apiUser = toUser(t.Context(), user1, false, false)
 	assert.False(t, apiUser.IsAdmin)
-	assert.Equal(t, api.UserVisibilityPublic, apiUser.Visibility)
+	assert.Equal(t, api.VisibilityStringPublic, apiUser.Visibility)
 
 	user31 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 31, IsAdmin: false, Visibility: api.VisibleTypePrivate})
 
 	apiUser = toUser(t.Context(), user31, true, true)
 	assert.False(t, apiUser.IsAdmin)
-	assert.Equal(t, api.UserVisibilityPrivate, apiUser.Visibility)
+	assert.Equal(t, api.VisibilityStringPrivate, apiUser.Visibility)
 }