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/static/common.js

@@ -0,0 +1,11 @@
+"use strict";
+
+// Language dropdown: remember the choice for a year and reload
+const langSelect = document.getElementById("lang");
+if (langSelect) {
+  langSelect.addEventListener("change", () => {
+    const secure = location.protocol === "https:" ? "; Secure" : "";
+    document.cookie = `lang=${encodeURIComponent(langSelect.value)}; Path=/; Max-Age=31536000; SameSite=Lax${secure}`;
+    location.reload();
+  });
+}