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/tests/users_bulk_secondary.spec.ts

@@ -112,7 +112,7 @@ test.describe("Users Bulk Upload Secondary Emails", () => {
 
     await page.getByTestId("bulk-start-btn").click();
 
-    await expect(page.getByText(/성공|Success/i)).toBeVisible();
+    await expect(page.getByText(/성공|Success/i).first()).toBeVisible();
 
     expect(bulkPayload).not.toBeNull();
     expect(bulkPayload.users).toHaveLength(1);