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

frontend/src/features/artifact/instances/pages/InstancesManagementPage.tsx

@@ -421,7 +421,7 @@ const InstancesManagementPage: React.FC = () => {
                         </p>
                       </div>
                     </div>
-                    <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
+                    <div className="grid grid-cols-1 md:grid-cols-2 xl:grid-cols-3 gap-4">
                       {instances.map((instance) => (
                         <InstanceCard
                           key={instance.id}
@@ -438,7 +438,7 @@ const InstancesManagementPage: React.FC = () => {
                 );
               })
             ) : (
-              <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
+              <div className="grid grid-cols-1 md:grid-cols-2 xl:grid-cols-3 gap-4">
                 {filteredInstances.map(({ instance }) => (
                 <InstanceCard
                   key={instance.id}