feat(user): support fixed UUID registration and enhance bulk import results

- Added support for fixed UUIDs during bulk registration (Search-first + ExternalID mapping)
- Implemented idempotency and visibility restoration for soft-deleted users
- Enhanced bulk upload UI to show 'New/Updated/Unchanged' status and modified fields
- Added logic to reclaim identifiers (login_id) from colliding records
- Added frontend E2E and backend unit tests for UUID integrity and conflict handling
- Fixed i18n, formatting, and mock tests to satisfy code-check
- Applied 'go fix' for 'omitzero' tags and general Go standards

adminfront/src/lib/adminApi.ts

@@ -725,6 +725,8 @@ export type BulkUserAppointment = {
 };
 
 export type BulkUserItem = {
+  id?: string;
+  uuid?: string;
   email: string;
   loginId?: string;
   name: string;
@@ -761,6 +763,7 @@ export type BulkUserResult = {
   success: boolean;
   message?: string;
   userId?: string;
+  modifiedFields?: string[];
 };
 
 export type BulkUserResponse = {