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