fix(storefront/a11y): raise sub-AA text contrast in pagination, cart line, cart summary, product gallery

[filiphsps]

What to build

Several user-visible labels fall below WCAG AA contrast on white/light surfaces:

• pagination.tsx:110,132 — text-gray-400 on white for disabled prev/next (≈3.1:1, fails AA 4.5:1 for normal text)
• cart-line.tsx:119,138 — text-gray-500 text-xs for strikethrough price and unit price on white
• cart-summary.tsx:233 — text-xs opacity-75 shipping disclaimer (small text + opacity reduction compounds)
• product-gallery.tsx:173 — text-gray-500 opacity-80 on bg-gray-100 for the alt-text badge

Raise to at least text-gray-600 (or design-token equivalent) and drop the opacity tricks. Keep the disabled visual distinct from enabled — outline style change or muted-but-readable color, not invisible.

Acceptance criteria

• Every listed selector hits ≥4.5:1 contrast against its actual background (verify with Chrome DevTools Color Picker contrast widget)
• Disabled pagination still reads as disabled visually (border weight or icon opacity, not text-color trick)
• Lighthouse / axe contrast audits on cart and PDP no longer flag these elements

Blocked by

None — can start immediately.

References

• apps/storefront/src/components/actionable/pagination.tsx:110,132
• apps/storefront/src/components/cart/cart-line.tsx:119,138
• apps/storefront/src/components/cart/cart-summary.tsx:233
• apps/storefront/src/components/products/product-gallery.tsx:173