Pass e2e credentials via environment variables

Remove login_user helper and LOGIN_PASSWORD constant, pass E2E_USER
and E2E_PASSWORD from the test script to playwright via env vars.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/test-e2e.sh

@@ -44,4 +44,6 @@ if ! curl -sf --max-time 5 "$GITEA_TEST_SERVER_URL/api/v1/users/$E2E_USER" > /de
 fi
 
 export GITEA_TEST_SERVER_URL
+export E2E_USER
+export E2E_PASSWORD
 exec pnpm exec playwright test "$@"