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>

tools/test-e2e.sh

@@ -43,6 +43,7 @@ LEVEL = Warn
 EOF
 
 export GITEA_WORK_DIR="$WORK_DIR"
+export GITEA_TEST_E2E=true
 
 # Start Gitea server
 echo "Starting Gitea server on port $FREE_PORT (workdir: $WORK_DIR)..."