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/shared/components/layout/SidebarLayout/SidebarNav.tsx

@@ -53,10 +53,10 @@ export default function SidebarNav({ items = [] as NavItem[], isOpen = true, onC
       <div key={item.key}>
         <button
           onClick={() => handleItemClick(item, hasChildren)}
-          className={`w-full text-left flex items-center gap-2 px-3 py-2 rounded-xl text-sm font-medium transition-colors duration-200 ${
+          className={`w-full text-left flex items-center gap-2 px-3 py-2 rounded-xl text-sm font-medium transition-colors duration-150 hover:bg-blue-50 dark:hover:bg-blue-900/20 ${
             item.active
               ? "bg-blue-50 text-blue-700 border border-blue-200 shadow-sm"
-              : "text-slate-600 hover:text-slate-950 hover:bg-slate-100"
+              : "text-slate-600 hover:text-slate-950"
           }`}
           style={{ paddingLeft: `${12 + level * 16}px` }}
         >