[Enhancement] Accessibility: certificate verification page reflow at <420px

What

The cert/verify.php page renders a meta row that flex-wraps oddly on viewports below 420px (typical small phone in portrait). The verification id and the verify URL stack awkwardly. Needs a stacked-on-mobile breakpoint to meet WCAG 1.4.10 (Reflow).

Where it lives

• Surface: the public certificate verification page (/learn/cert/{id})
• Backlog id: B-A11Y-2

Implementation notes

• Add a @media (max-width: 420px) block that switches .cert-meta from flex-row to flex-column with full-width children
• Verify with the same WCAG 1.4.10 test (200% zoom, no horizontal scroll)

Tracked as B-A11Y-2 in docs/BACKLOG.md.