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>
