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/cmd/api/main.go

@@ -285,6 +285,8 @@ func setupRouter(
 	protected.HandleFunc("/clusters/{cluster_id}/instances/{instance_id}/entries", instanceHandler.ListInstanceEntries).Methods(http.MethodGet)
 	protected.HandleFunc("/clusters/{cluster_id}/instances/{instance_id}/diagnostics", instanceHandler.GetInstanceDiagnostics).Methods(http.MethodGet)
 	protected.HandleFunc("/clusters/{cluster_id}/instances/{instance_id}/logs/stream", instanceHandler.StreamInstanceLogs).Methods(http.MethodGet)
+	protected.HandleFunc("/clusters/{cluster_id}/instances/{instance_id}/scale", instanceHandler.ScaleInstance).Methods(http.MethodPost)
+	protected.HandleFunc("/clusters/{cluster_id}/instances/{instance_id}/values-diff", instanceHandler.GetInstanceValuesDiff).Methods(http.MethodGet)
 
 	// ===== Monitoring 路由 =====
 	protected.HandleFunc("/monitoring/clusters", monitoringHandler.ListClusterMonitoring).Methods(http.MethodGet)