fix: make Admin LDAP sign-in work against GLAuth

Three bugs blocked sign-in entirely:

- Login.razor is static-SSR but its form model lacked
  [SupplyParameterFromForm], so the posted username/password never
  bound — SignInAsync saw empty fields and bailed before LDAP was
  contacted. Annotate the model; seed it in OnInitialized since
  BL0008 forbids an initializer on a [SupplyParameterFromForm]
  property.
- appsettings.json ServiceAccountDn used ou=svcaccts, which GLAuth
  reads as a (non-existent) group — the service-account bind failed
  with "Group not found". Use cn=serviceaccount,dc=lmxopcua,dc=local.
- LdapAuthService resolved the user DN by searching (uid=...), but
  GLAuth keys users by cn. Add an LdapOptions.UserNameAttribute knob
  (default cn for GLAuth; set sAMAccountName for Active Directory)
  and use it for the search filter.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Server/ZB.MOM.WW.OtOpcUa.Admin/Security/LdapOptions.cs

@@ -29,6 +29,13 @@ public sealed class LdapOptions
     public string DisplayNameAttribute { get; set; } = "cn";
     public string GroupAttribute { get; set; } = "memberOf";
 
+    /// <summary>
+    ///     Attribute the service-account search matches the login name against to resolve the
+    ///     user's DN. <c>cn</c> for GLAuth (the dev default); set <c>sAMAccountName</c> for
+    ///     Active Directory.
+    /// </summary>
+    public string UserNameAttribute { get; set; } = "cn";
+
     /// <summary>
     ///     Maps LDAP group name → Admin role. Group match is case-insensitive. A user gets every
     ///     role whose source group is in their membership list. Example dev mapping: