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/api_key_handler_test.go

@@ -26,7 +26,7 @@ func TestApiKeyHandler_CreateApiKey(t *testing.T) {
 
 	app.Post("/api-keys", h.CreateApiKey)
 
-	input := map[string]interface{}{
+	input := map[string]any{
 		"name":   "M2M Test",
 		"scopes": []string{"read", "write"},
 	}
@@ -47,7 +47,7 @@ func TestApiKeyHandler_Validation(t *testing.T) {
 	app.Post("/api-keys", h.CreateApiKey)
 
 	// Missing name
-	input := map[string]interface{}{
+	input := map[string]any{
 		"scopes": []string{"read"},
 	}
 	body, _ := json.Marshal(input)
@@ -65,7 +65,7 @@ func TestApiKeyHandler_UpdateApiKeyScopesRequiresDatabase(t *testing.T) {
 
 	app.Patch("/api-keys/:id", h.UpdateApiKey)
 
-	body, _ := json.Marshal(map[string]interface{}{
+	body, _ := json.Marshal(map[string]any{
 		"scopes": []string{"org-context:read"},
 	})
 	req := httptest.NewRequest("PATCH", "/api-keys/api-key-id", bytes.NewReader(body))