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>

tests/e2e/utils.ts

@@ -104,6 +104,10 @@ export async function login(page: Page, username = env.GITEA_TEST_E2E_USER, pass
   await expect(page.getByRole('link', {name: 'Sign In'})).toBeHidden();
 }
 
+export async function assertNoJsError(page: Page) {
+  await expect(page.locator('.js-global-error')).toHaveCount(0);
+}
+
 export async function logout(page: Page) {
   await page.context().clearCookies(); // workaround issues related to fomantic dropdown
   await page.goto('/');