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>

frontend/messages/zh-TW.json

@@ -149,6 +149,7 @@
   },
   "Admin": {
     "user_management": "使用者管理",
+    "ldap": "LDAP",
     "avatar": "頭像",
     "id": "ID",
     "email": "電子郵件",
@@ -434,5 +435,27 @@
     "tag_error_create": "標籤建立失敗，請重試。",
     "tag_error_update": "標籤更新失敗，請重試。",
     "tag_error_delete": "標籤刪除失敗，請重試。"
+  },
+  "Ldap": {
+    "ldap_settings": "LDAP 設定",
+    "ldap_description": "透過 LDAP 目錄驗證使用者身分。",
+    "enable_ldap": "啟用 LDAP",
+    "enable_ldap_desc": "允許使用者使用目錄憑證登入。",
+    "server_url": "伺服器位址",
+    "bind_dn": "繫結 DN",
+    "bind_credentials": "繫結密碼",
+    "bind_credentials_placeholder": "留空以保留目前密碼",
+    "search_base": "搜尋基準",
+    "search_filter": "搜尋過濾器",
+    "email_attribute": "電子郵件屬性",
+    "username_attribute": "使用者名稱屬性",
+    "save": "儲存",
+    "saved": "已儲存 LDAP 設定",
+    "save_error": "儲存 LDAP 設定失敗",
+    "test_connection": "測試連線",
+    "test_username": "測試使用者名稱",
+    "test_password": "測試密碼",
+    "test_success": "連線成功",
+    "test_failed": "連線失敗"
   }
 }