add TypeScript interfaces for cart and cart items

Introduces BaseCartItem, MovieCartItem, SnackCartItem, and the CartItem
discriminated union in bigConstants.ts. Replaces all any-typed cart
references across CartView, CheckoutView, SnacksView, and BookingModal
with the new typed interfaces. Also types users/currentUser with the
existing User interface and removes the unused JSONType import.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/SnacksView.astro

@@ -450,6 +450,7 @@
 
 <script>
     import { cart, updateCart } from "../scripts/bigConstants";
+    import type { SnackCartItem } from "../scripts/bigConstants";
 
     const snacksView = document.getElementById("snacks-view");
 
@@ -471,7 +472,7 @@
         const activeOption = snackCard.querySelector(".opt-btn.active") as HTMLElement;
         const variantVal = activeOption ? activeOption.innerText : "Normal";
 
-        cart.push({
+        const snackItem: SnackCartItem = {
             id: Date.now() + Math.random(),
             category: "snack",
             title: snackTitle,
@@ -480,7 +481,8 @@
             type: "SNACK",
             price: priceVal,
             img: snackImg
-        });
+        };
+        cart.push(snackItem);
 
         updateCart(cart);
         window.dispatchEvent(new CustomEvent("cart-updated"));