feat(auth)!: MxGateway canonical dashboard roles — Admin→Administrator (Task 1.7)

Standardize the dashboard role VALUE on the canonical six: Admin→Administrator
(Viewer unchanged). Pure value rename via DashboardRoles.Admin constant +
appsettings GroupToRole; the GatewayOptionsValidator allowed-set/message track
the constant so they now require 'Administrator' or 'Viewer'. Enforcement is
unchanged — Administrator authorizes exactly what Admin did.

Dashboard roles are derived at login from LDAP groups via GroupToRole and are
never persisted to the SQLite auth store, so no DB migration/seed change.

UNTOUCHED: the separate gRPC API-key scope GatewayScopes.Admin = "admin"
(lowercase) and every "admin" scope literal — a distinct data-plane system.

src/ZB.MOM.WW.MxGateway.Server/appsettings.json

@@ -60,7 +60,7 @@
       "RecentSessionLimit": 200,
       "ShowTagValues": false,
       "GroupToRole": {
-        "GwAdmin": "Admin",
+        "GwAdmin": "Administrator",
         "GwReader": "Viewer"
       }
     },