Fix relative-time RangeError (#37021)

`navigator.language` can be `undefined` in headless browsers (e.g.
Playwright Firefox), causing `RangeError: invalid language tag:
"undefined"` in `Intl.DateTimeFormat` within the `relative-time` web
component.

Also adds an e2e test that verifies `relative-time` renders correctly
and a shared `assertNoJsError` helper.

Bug is als present in https://github.com/github/relative-time-element
but (incorrectly) masked there.

Fixes: https://github.com/go-gitea/gitea/issues/25324

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/web.go

@@ -1740,7 +1740,7 @@ func registerWebRoutes(m *web.Router, webAuth *AuthMiddleware) {
 		m.Get("/swagger.v1.json", SwaggerV1Json)
 	}
 
-	if !setting.IsProd {
+	if !setting.IsProd || setting.IsInE2eTesting() {
 		m.Group("/devtest", func() {
 			m.Any("", devtest.List)
 			m.Any("/fetch-action-test", devtest.FetchActionTest)