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/appsettings.json

@@ -10,7 +10,7 @@
       "UseTls": false,
       "AllowInsecureLdap": true,
       "SearchBase": "dc=lmxopcua,dc=local",
-      "ServiceAccountDn": "cn=serviceaccount,ou=svcaccts,dc=lmxopcua,dc=local",
+      "ServiceAccountDn": "cn=serviceaccount,dc=lmxopcua,dc=local",
       "ServiceAccountPassword": "serviceaccount123",
       "DisplayNameAttribute": "cn",
       "GroupAttribute": "memberOf",