feat: scale instances, --reuse-values, values diff, UI redesign, hover animations

Backend (Phase 1):
- Add ScaleInstance endpoint (POST /clusters/{id}/instances/{id}/scale)
- Add GetInstanceValuesDiff endpoint (GET .../values-diff)
- Enable ReuseValues=true in Helm Upgrade for --reuse-values behavior
- Add GetValues/GetChartDefaultValues to HelmClient interface
- Add ScaleInstanceRequest/Response and InstanceValuesDiffResponse DTOs

Frontend (Phase 2):
- InstanceCard: +/- scale buttons with loading spinner
- ModifyModal: values diff view (current vs defaults), Use Defaults button
- ArtifactBrowserPage: collapsible sidebar, compact tag grid, search filter
- TagCard: "LATEST" badge, compact layout, responsive design
- InstanceCard: compact 3-column layout, fewer scrolls needed
- InstancesManagementPage: 3-column grid, compact view
- Global hover-lift and hover-glow CSS utilities
- SidebarNav: subtle hover transition on links

backend/internal/domain/service/instance_service_test.go

@@ -167,5 +167,9 @@ func (*stubHelmClient) GetValues(ctx context.Context, cluster *entity.Cluster, r
 	return nil, nil
 }
 
+func (*stubHelmClient) GetChartDefaultValues(chartPath string) (map[string]interface{}, error) {
+	return nil, nil
+}
+
 var _ repository.ClusterRepository = (*stubClusterRepo)(nil)
 var _ repository.HelmClient = (*stubHelmClient)(nil)