fix(webapp): incorrect user domain parsing for UPN format (#1431)

Fix username parsing logic to correctly handle UPN and down-level
formats.
Only splits DOMAIN\user format; preserves user@domain.com as-is.

Issue: DGW-297

Author: kristahouse

webapp/src/client/app/shared/services/utils/string.service.ts

@@ -17,7 +17,7 @@ export class StringService {
     this.utils = parent;
   }
 
-  //DOMAIN\username or username@DOMAIN
+  //DOMAIN\username
   extractDomain(fullUsername: string): ExtractedUsernameDomain {
     const extractionData: ExtractedUsernameDomain = {
       username: fullUsername,
@@ -27,9 +27,6 @@ export class StringService {
     if (fullUsername.includes('\\')) {
       extractionData.domain = fullUsername.split('\\')[0];
       extractionData.username = fullUsername.split('\\')[1];
-    } else if (fullUsername.includes('@')) {
-      extractionData.domain = fullUsername.split('@')[1];
-      extractionData.username = fullUsername.split('@')[0];
     }
     return extractionData;
   }