feat(images): frontend Images page, catalog hooks, download dialog

Phase 1 UI for VM image mirror feature:
- use-image-catalog hook (catalog, lxc repo templates, target storages, download)
- ImagesPage: server/node context, arch/search filters, cloud + LXC sections
- DownloadImageDialog: storage picker, background download via DeployTask
- CatalogImage/ImageMirror types, image_download/image_template task kinds
- sidebar nav 'Images' + /images route

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/layouts/AppLayout.tsx

@@ -5,6 +5,7 @@ import {
   Monitor,
   Network,
   HardDrive,
+  HardDriveDownload,
   Archive,
   Globe,
   Settings,
@@ -61,6 +62,7 @@ const navGroups: NavGroup[] = [
       { label: 'Nodes', icon: Server, path: '/nodes' },
       { label: 'Cluster', icon: Network, path: '/cluster' },
       { label: 'Templates', icon: HardDrive, path: '/templates' },
+      { label: 'Images', icon: HardDriveDownload, path: '/images' },
       { label: 'Backups', icon: Archive, path: '/backups' },
       { label: 'Tasks', icon: ClipboardList, path: '/tasks' },
     ],