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

backend/internal/logger/logger_test.go

@@ -23,7 +23,6 @@ func TestResolveBackendLogLevel_DefaultsByAppEnv(t *testing.T) {
 	}
 
 	for _, tc := range testCases {
-		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()