feat: add LDAP authentication settings and toggle

Add an admin-only LDAP configuration UI with an enable toggle and full
sign-in integration.

Backend:
- ldapSettings model + migration (single-row config)
- GET/PUT/test routes under /ldap (admin-gated; bind password masked)
- shared ldapClient with RFC 4515 filter escaping and empty-password guard
- signin tries local auth first, then LDAP when enabled (find-or-create
  local user) so the bootstrap admin is never locked out

Frontend:
- LDAP settings page (Switch + form + test connection) under /admin/ldap
- AdminNav tabs between user management and LDAP
- ldapControl util, types, and Ldap i18n namespace for all 6 locales

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routes/ldap/index.js

@@ -0,0 +1,33 @@
+import express from 'express';
+import { DataTypes } from 'sequelize';
+import defineLdapSetting from '../../models/ldapSettings.js';
+import authMiddleware from '../../middleware/auth.js';
+import { getLdapSettings } from './ldapClient.js';
+const router = express.Router();
+
+// Never expose the stored bind password. Report only whether one is set.
+function serialize(settings) {
+  const json = settings.toJSON();
+  json.bindCredentialsSet = Boolean(json.bindCredentials);
+  json.bindCredentials = '';
+  return json;
+}
+
+export default function (sequelize) {
+  const { verifySignedIn, verifyAdmin } = authMiddleware(sequelize);
+  const LdapSetting = defineLdapSetting(sequelize, DataTypes);
+
+  router.get('/', verifySignedIn, verifyAdmin, async (req, res) => {
+    try {
+      const settings = await getLdapSettings(LdapSetting);
+      res.json(serialize(settings));
+    } catch (error) {
+      console.error(error);
+      res.status(500).send('Internal Server Error');
+    }
+  });
+
+  return router;
+}
+
+export { serialize };