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/models/ldapSettings.js

@@ -0,0 +1,49 @@
+function defineLdapSetting(sequelize, DataTypes) {
+  const LdapSetting = sequelize.define('LdapSetting', {
+    enabled: {
+      type: DataTypes.BOOLEAN,
+      allowNull: false,
+      defaultValue: false,
+    },
+    url: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: '',
+    },
+    bindDn: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: '',
+    },
+    bindCredentials: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: '',
+    },
+    searchBase: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: '',
+    },
+    // {{username}} is replaced with the value typed in the sign-in form.
+    searchFilter: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: '(mail={{username}})',
+    },
+    emailAttribute: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: 'mail',
+    },
+    usernameAttribute: {
+      type: DataTypes.STRING,
+      allowNull: false,
+      defaultValue: 'cn',
+    },
+  });
+
+  return LdapSetting;
+}
+
+export default defineLdapSetting;