feat(security): scan comparison UI (#648)

Side-by-side vulnerability scan comparison with two entry points:

- Compare button plus inline baseline picker inside the scan drawer.
- New Scan History page reachable from the Resources Hub, grouping
  completed scans by image with a checkbox selection flow.

The comparison sheet shows a severity delta ribbon, Added/Removed/Unchanged
filters, and a paginated CVE table. Cross-image comparisons are allowed
but flagged with a warning. Compare access is gated to Skipper and
Admiral tiers; the underlying /security/compare endpoint is unchanged.

frontend/src/components/NodeManager.tsx

@@ -24,8 +24,8 @@ interface NodeSchedulingSummary {
 
 export const SENCHO_NAVIGATE_EVENT = 'sencho-navigate';
 export interface SenchoNavigateDetail {
-  view: 'scheduled-ops' | 'auto-updates';
-  nodeId: number;
+  view: 'scheduled-ops' | 'auto-updates' | 'security-history';
+  nodeId?: number;
 }
 
 function formatRelativeTime(timestamp: number): string {