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/package.json

@@ -18,6 +18,7 @@
     "express": "^4.21.0",
     "express-rate-limit": "^7.4.1",
     "jsonwebtoken": "^9.0.2",
+    "ldapts": "^8.1.8",
     "multer": "^1.4.5-lts.1",
     "papaparse": "^5.5.2",
     "sequelize": "^6.37.7",