Improve cart and dark mode UX

- Clear cart when customer logs out
- Hide cart bag button for non-authenticated users (only show for logged-in customers)
- Fix dark mode text visibility in all form inputs (input/textarea/select elements)

Addresses user request: cart should clear on logout and only be accessible to logged-in customers.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/components/Header.tsx

@@ -77,7 +77,7 @@ export default async function Header() {
             <AccountMenu customer={customer ? { name: customer.name, email: customer.email } : null} />
           </div>
           <ThemeToggle />
-          <CartButton />
+          {customer && <CartButton />}
         </div>
       </div>
     </header>