Wire up product variants end-to-end, add tracking-number display

Completes the frontend half of the Payload backend's variant/inventory/
tracking work (see that repo's own commit):

- **Cart**: CartItem gained an optional `variant?: string` field — every
  function that used to match a line by `id` alone (addToCart/
  removeFromCart/setQuantity) now matches by `(id, variant)` together via
  a shared sameLine() helper, so two lines for the same product with
  different variants stay separate entries. `variant` undefined on both
  sides (the no-variants case) still matches by simple equality, so every
  pre-existing call site keeps working unchanged.
- **Selection UI**: AddToCartInlineButton renders a <select> above the
  button when given a non-empty `variants` prop (ProductGrid/
  RelatedProducts pass product.variants straight through); defaults to
  the first variant.
- **Pricing**: cartTotals.ts's new effectivePrice(entry, product) — a
  variant's priceOverride wins over the base product price. Every cart/
  checkout/order-confirmation total and per-line price display now goes
  through this instead of reading product.price directly (fixes both a
  wrong-price bug and a duplicate-React-key bug the old `key={product.id}`
  pattern would have had the moment two variants of one product were both
  in the cart).
- **Checkout**: re-validates the requested variant server-side (same
  "never trust the client" reasoning as price re-derivation) — a variant
  name that doesn't exist on that product fails the whole checkout.
  variantName snapshots onto orders.items, shown as a parenthetical next
  to the product name on the confirmation email, both invoice PDF types,
  and the order-detail page.
- **Cross-device cart**: Customers.cart[].variantName (synced via
  /api/account/cart) carries the selection through a login/logout cycle,
  not just the current session.

Also adds tracking-number display: /konto/bestellungen/[orderNumber]
shows a clickable link when orders.trackingNumber is set, built by a new
app/lib/tracking.ts that mirrors the Payload backend's own copy
byte-for-byte close (same carrier set/URL patterns) so what a customer
sees here matches exactly what the order-shipped email already links to.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/cartTotals.ts

@@ -6,15 +6,26 @@ import type { Product } from "./payload";
 // discount-code math to all three at once would otherwise mean hand-editing
 // 3 near-identical blocks (and risking them drifting apart).
 
-export type CartLine = { entry: { qty: number }; product: Product };
+export type CartLine = { entry: { qty: number; variant?: string }; product: Product };
 export type DiscountLike = { type: "percent" | "fixed"; value: number };
 
+// A selected variant's priceOverride wins over the base product price —
+// null/undefined priceOverride (or no variant selected at all) falls back
+// to it. The one place cart/checkout math needs to know about variants at
+// all; every other total below builds on this instead of `product.price`
+// directly.
+export function effectivePrice(entry: { variant?: string }, product: Product): number {
+  if (!entry.variant) return product.price;
+  const variant = product.variants.find((v) => v.name === entry.variant);
+  return variant?.priceOverride ?? product.price;
+}
+
 // Split out from computeCartTotals() below because callers need a subtotal
 // figure *before* they can decide a shipping cost (e.g. checking it against
 // a free-shipping threshold) — which computeCartTotals itself takes as an
 // input, not something it can decide on its own.
 export function computeSubtotal(items: CartLine[]): number {
-  return items.reduce((sum, { entry, product }) => sum + entry.qty * product.price, 0);
+  return items.reduce((sum, { entry, product }) => sum + entry.qty * effectivePrice(entry, product), 0);
 }
 
 export type CartTotals = {