Initial version of fail2ban-webui

Web interface to view, ban and unban fail2ban IPs on several hosts via
SSH, generalised from the private f2b-web tool.

- Web UI (Flask/Waitress): hosts.yaml, local/OIDC/proxy authentication,
  English/German UI, origin map, optional Loki/Grafana/GeoIP, CLI helpers
- Agent f2b-webctl with install.sh/uninstall.sh (dedicated user + sudo,
  from= restriction, protected networks, manual jail)
- Hardened Docker image, examples, documentation, smoke test, testbed

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/webui/templates/error.html

@@ -0,0 +1,12 @@
+{% extends "base.html" %}
+{% block title %}{{ t('error_title', code=code) }} – {{ t('title') }}{% endblock %}
+
+{% block content %}
+<main class="login">
+  <section class="panel">
+    <h2>{{ t('error_title', code=code) }}</h2>
+    <p class="msg error">{{ message }}</p>
+    <p><a href="{{ url_for('index') }}">{{ t('error_back') }}</a></p>
+  </section>
+</main>
+{% endblock %}