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/vitest.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/__tests__/setup.ts'],
+    include: ['src/**/*.{test,spec}.{ts,tsx}'],
+    css: false,
+  },
+});