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/handler/dev_handler_rp_metadata_test.go

@@ -35,10 +35,10 @@ func (m *devMockRPUserMetadataRepo) Upsert(ctx context.Context, metadata *domain
 func TestDevHandler_RPUserMetadataRoundTrip(t *testing.T) {
 	transport := roundTripFunc(func(r *http.Request) (*http.Response, error) {
 		if r.URL.Path == "/clients/client-1" {
-			return httpJSONAny(r, http.StatusOK, map[string]interface{}{
+			return httpJSONAny(r, http.StatusOK, map[string]any{
 				"client_id":   "client-1",
 				"client_name": "Client One",
-				"metadata": map[string]interface{}{
+				"metadata": map[string]any{
 					"tenant_id": "tenant-1",
 				},
 			}), nil