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/adapter/output/helm/mock/helm_client_mock.go

@@ -194,3 +194,13 @@ func (c *HelmClientMock) GetValues(ctx context.Context, cluster *entity.Cluster,
 	return instance.Values, nil
 }
 
+func (c *HelmClientMock) GetChartDefaultValues(chartPath string) (map[string]interface{}, error) {
+	return map[string]interface{}{
+		"replicaCount": 1,
+		"image": map[string]interface{}{
+			"repository": "nginx",
+			"tag":        "latest",
+		},
+	}, nil
+}
+