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/utils/ldapControl.ts

@@ -0,0 +1,57 @@
+import { LdapSettingsType } from '@/types/ldap';
+import Config from '@/config/config';
+import { logError } from '@/utils/errorHandler';
+const apiServer = Config.apiServer;
+
+async function fetchLdapSettings(jwt: string): Promise<LdapSettingsType> {
+  const response = await fetch(`${apiServer}/ldap`, {
+    method: 'GET',
+    headers: {
+      'Content-Type': 'application/json',
+      Authorization: `Bearer ${jwt}`,
+    },
+  });
+  if (!response.ok) {
+    throw new Error(`HTTP error! Status: ${response.status}`);
+  }
+  return response.json();
+}
+
+async function updateLdapSettings(jwt: string, settings: LdapSettingsType): Promise<LdapSettingsType> {
+  const response = await fetch(`${apiServer}/ldap`, {
+    method: 'PUT',
+    headers: {
+      'Content-Type': 'application/json',
+      Authorization: `Bearer ${jwt}`,
+    },
+    body: JSON.stringify(settings),
+  });
+  if (!response.ok) {
+    throw new Error(`HTTP error! Status: ${response.status}`);
+  }
+  return response.json();
+}
+
+async function testLdapConnection(
+  jwt: string,
+  settings: LdapSettingsType,
+  testUsername: string,
+  testPassword: string
+): Promise<{ ok: boolean; message?: string }> {
+  try {
+    const response = await fetch(`${apiServer}/ldap/test`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Authorization: `Bearer ${jwt}`,
+      },
+      body: JSON.stringify({ ...settings, testUsername, testPassword }),
+    });
+    return response.json();
+  } catch (error: unknown) {
+    logError('Error testing LDAP connection:', error);
+    return { ok: false, message: 'Request failed' };
+  }
+}
+
+export { fetchLdapSettings, updateLdapSettings, testLdapConnection };