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/Settings.tsx

@@ -153,7 +153,7 @@ export function Settings() {
               {copied ? 'Copied!' : 'Copy'}
             </button>
           </div>
-          <p className="mt-2 text-xs text-gray-400">
+          <p className="mt-2 text-xs text-gray-500">
             Supports Meijer, Kroger, and Target receipt emails.
           </p>
         </div>