Discount codes:
- New shared lib/cartTotals.ts (computeSubtotal/computeCartTotals) factored
out of the previously-triplicated subtotal/totalSavings/total math in
CartContent/CheckoutContent/BestellbestaetigungContent, extended to also
fold in a discount amount (percent or fixed, clamped so total can't go
negative).
- lib/discount.ts mirrors lib/cart.ts's exact localStorage pattern so an
applied code survives the /cart -> /checkout transition without a second
input field — Checkout only displays it.
- New /api/discount/validate (read-only check) and /api/discount/redeem
(re-validates + increments the redemption counter, called once from
checkout's handlePurchase right before the OrderSnapshot is written).
Both talk to Payload's new discount-codes collection through
lib/discountServer.ts, a server-only module kept separate from
lib/payload.ts on purpose (that file is also imported by "use client"
components; the RSC-boundary break hit earlier this session was exactly
this mistake with next/headers).
- OrderSnapshot gains discountCode/discountAmount so /bestellbestaetigung
displays what was actually applied instead of losing it on recompute.
RelatedProducts: no longer falls back to re-suggesting a product already
in the cart just to pad the grid out to 3 cards — shows only the
genuinely available remainder (down to 1 card), centered in the 12-column
grid instead of left-aligned.
Marco