Setting: notify of failed logins only for existing accounts (codeling/bfstop#172)

[codeling]

Adds the notifyFailedOnlyExisting setting (default disabled) to the Notification tab of the settings form. The behaviour lives in the plugin: see the companion PR in codeling/bfstop for #172.

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https://claude.ai/code/session_013HDvAhe4W1FxjobS8Etch2

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[codeling]

The only red check is Manifest, language and package checks, and it is not caused by this PR. It fails in its first step while installing libxml2-utils: E: Failed to fetch mirror+file:/etc/apt/apt-mirrors.txt/.../libxml2-utils_2.9.14+dfsg-1.3ubuntu3.9_amd64.deb 404 Not Found. That is a stale package index on the runner, so none of the repo's own checks ran. I re-ran the job once and it failed in about 10 seconds again, consistent with the same step. I confirmed the identical apt 404 in the first run's log; I did not open the re-run's log for this repo.

The only file changed here is admin/forms/settings.xml; I ran xmllint --noout on it locally and it is well-formed. The other 6 checks are green.

No fix exists yet. A small change in .github/workflows/ci.yml would resolve it: run sudo apt-get update before sudo apt-get install -y -q libxml2-utils. I have not added that here, to keep this PR scoped. The same failure shows on the companion PR codeling/bfstop#238.

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