fix: improve color contrast for accessibility compliance (#222)

- Changed text-gray-400 to text-gray-500 in Dashboard, StoreComparison,
  Purchases, Settings, Alerts, and Coupons pages
- text-gray-500 (#6b7280) has 4.6:1 contrast ratio on white, meeting WCAG AA
- text-gray-400 (#99a1af) only had 2.6:1, failing axe-core accessibility checks

Co-authored-by: Test User <test@example.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

src/pages/Coupons.tsx

@@ -62,7 +62,7 @@ export function Coupons() {
                   <p className="mt-0.5 text-xs text-gray-500">{coupon.storeName}</p>
                   <p
                     className={`mt-1 text-xs ${
-                      expiringSoon ? 'font-medium text-orange-600' : 'text-gray-400'
+                      expiringSoon ? 'font-medium text-orange-600' : 'text-gray-500'
                     }`}
                   >
                     Expires{' '}