fix(BUY-63740): Fix React hydration errors on SEO landing pages

[BuyWhere]

Summary

Fixes React hydration errors (minified errors #418/#422) on SEO landing pages in production.

Root Cause

Two distinct issues:

1. Nested interactive controls in ProductGridCard — <a> inside Next.js <Link> violates React's rule against nested interactive elements
2. Date render-time mismatch in SeoLandingPage — new Date().toLocaleDateString() produced different values on server vs. client

Changes

ProductGridCard.tsx

• Replaced inner <a href target=_blank> with <span role=button tabIndex=0> + click/keyboard handlers using window.open()
• Maintains same UX: opens merchant link in new tab with noopener,noreferrer

SeoLandingPage.tsx

• Replaced new Date().toLocaleDateString() fallback with stable string "Live prices updated regularly"
• Avoids hydration mismatch when no product updatedAt timestamp is available

Verification

VidMee renders against production URLs confirm:

• best-robot-vacuums-2026: No React errors (1 network failure is external image CDN)
• air-purifier-singapore: Zero console errors, zero network failures (except expected RSC prefetch)

Fixes BUY-63740