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/relative-time.test.ts

@@ -0,0 +1,10 @@
+import {test, expect} from '@playwright/test';
+import {assertNoJsError} from './utils.ts';
+
+test('relative-time renders without errors', async ({page}) => {
+  await page.goto('/devtest/relative-time');
+  const relativeTime = page.getByTestId('relative-time-now');
+  await expect(relativeTime).toHaveAttribute('data-tooltip-content', /.+/);
+  await expect(relativeTime).toHaveText('now');
+  await assertNoJsError(page);
+});