Notify of failed logins only for existing accounts (#172)

[codeling]

Implements #172.

The failed login notification mail is only sent if the new setting notifyFailedOnlyExisting is off, or the username (or email address, as suggested in the issue comments) that was entered belongs to an existing account. The check uses the name as typed, since the stored entry only holds a hash for unknown names. Other attempts are still recorded and counted towards blocking.

The setting itself (default: disabled, so existing behaviour is unchanged) is added to the component's settings form in the companion PR in com_bfstop. Includes en-GB and de-DE strings and a changelog entry.

Not tested end to end: I did not run the integration test suite (needs a Joomla database); the PHP lints cleanly.

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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 the XML files are well-formed 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 identically.

I ran the same checks locally on this branch: xmllint over all XML files and php tests/lint/check-language.php bfstop.xml PLG_SYSTEM_BFSTOP both pass (the language lint only prints the existing missing-translation warnings). The other 8 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 to #172. The same failure shows on the companion PR codeling/com_bfstop#19.

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