test: expand exporter/css/a11y/i18n regressions and add e2e skip-link focus check

components/previewToggle.aria.snapshot.test.tsx

@@ -0,0 +1,31 @@
+import { render, screen } from '@testing-library/react'
+import '@testing-library/jest-dom'
+import React from 'react'
+import BuilderClientPage from '@/components/BuilderClientPage'
+import I18nProvider from '@/components/I18nProvider'
+import en from '@/messages/en.json'
+
+describe('Preview viewport toggle ARIA regression', () => {
+  it('Desktop/Mobile buttons expose stable ARIA attributes', () => {
+    render(
+      <I18nProvider locale="en" messages={en as unknown as Record<string, string>}>
+        <BuilderClientPage />
+      </I18nProvider>
+    )
+
+    const desktop = screen.getByRole('button', { name: 'Desktop' })
+    const mobile = screen.getByRole('button', { name: 'Mobile' })
+
+    // describedby shared help
+    expect(desktop).toHaveAttribute('aria-describedby', 'preview-toggle-help')
+    expect(mobile).toHaveAttribute('aria-describedby', 'preview-toggle-help')
+
+    // controls preview container
+    expect(desktop).toHaveAttribute('aria-controls', 'preview-container')
+    expect(mobile).toHaveAttribute('aria-controls', 'preview-container')
+
+    // default pressed state: Desktop true, Mobile false
+    expect(desktop).toHaveAttribute('aria-pressed', 'true')
+    expect(mobile).toHaveAttribute('aria-pressed', 'false')
+  })
+})