test(security): add scan comparison coverage (#656)

Backend supertest suite for GET /api/security/compare covers tier gating,
input validation, cross-node isolation, diff partitioning by
vulnerability_id::pkg_name, suppression application, and cross-image
comparison. Frontend vitest + React Testing Library scaffolding with
component tests for ScanComparisonSheet (loading, error recovery,
cross-image warning, filter pills, reload on id change) and
SecurityHistoryView (mount fetch, selection cap, oldest-first baseline
ordering, tier gating).

frontend/src/__tests__/setup.ts

@@ -0,0 +1,27 @@
+import '@testing-library/jest-dom/vitest';
+import { afterEach, vi } from 'vitest';
+import { cleanup } from '@testing-library/react';
+
+class MockResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+globalThis.ResizeObserver = globalThis.ResizeObserver ?? MockResizeObserver;
+
+if (typeof window !== 'undefined' && !window.matchMedia) {
+  window.matchMedia = vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  }));
+}
+
+afterEach(() => {
+  cleanup();
+});