fix(spa): use couponError to fix build warning

apps/spa/src/components/CartDrawer.tsx

@@ -207,21 +207,24 @@ function CartDrawerFooter({ subtotal, onClose, t }: { subtotal: number; onClose:
       
       {/* Coupon Input */}
       {!savedCoupon && (
-        <div className="flex gap-2">
-          <input
-            type="text"
-            value={couponInput}
-            onChange={e => setCouponInput(e.target.value.toUpperCase())}
-            placeholder={t('cart.couponPlaceholder', 'Discount Code')}
-            className="flex-1 bg-white border border-spa-stone/20 rounded-lg px-3 py-2 text-sm text-pop-black focus:border-spa-forest outline-none uppercase tracking-widest placeholder:text-spa-stone/50"
-          />
-          <button
-            onClick={handleApplyCoupon}
-            disabled={couponLoading || !couponInput.trim()}
-            className="px-4 bg-spa-stone/10 hover:bg-spa-stone/20 border border-spa-stone/10 rounded-lg text-xs font-bold uppercase tracking-widest disabled:opacity-50 transition-all text-pop-black"
-          >
-            {couponLoading ? '...' : t('cart.applyCoupon', 'Apply')}
-          </button>
+        <div className="flex flex-col gap-1">
+          <div className="flex gap-2">
+            <input
+              type="text"
+              value={couponInput}
+              onChange={e => setCouponInput(e.target.value.toUpperCase())}
+              placeholder={t('cart.couponPlaceholder', 'Discount Code')}
+              className="flex-1 bg-white border border-spa-stone/20 rounded-lg px-3 py-2 text-sm text-pop-black focus:border-spa-forest outline-none uppercase tracking-widest placeholder:text-spa-stone/50"
+            />
+            <button
+              onClick={handleApplyCoupon}
+              disabled={couponLoading || !couponInput.trim()}
+              className="px-4 bg-spa-stone/10 hover:bg-spa-stone/20 border border-spa-stone/10 rounded-lg text-xs font-bold uppercase tracking-widest disabled:opacity-50 transition-all text-pop-black"
+            >
+              {couponLoading ? '...' : t('cart.applyCoupon', 'Apply')}
+            </button>
+          </div>
+          {couponError && <span className="text-xs text-rose-500 font-medium">{couponError}</span>}
         </div>
       )}