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/payload.ts

@@ -170,6 +170,7 @@ export type Product = {
   spotlightHeadline: string | null;
   spotlightText: string | null;
   spotlightImage: string | null;
+  variants: { name: string; priceOverride: number | null }[];
 };
 
 type PayloadProduct = {
@@ -188,6 +189,7 @@ type PayloadProduct = {
   spotlightHeadline: string | null;
   spotlightText: string | null;
   spotlightImage: { url: string } | number | null;
+  variants: { name: string; priceOverride: number | null }[] | null;
 };
 
 // Shared by getProducts() and getPostBySlug()'s relatedProduct — kept in
@@ -211,6 +213,7 @@ export function mapPayloadProduct(product: PayloadProduct): Product {
     spotlightText: product.spotlightText || null,
     spotlightImage:
       typeof product.spotlightImage === "object" && product.spotlightImage ? product.spotlightImage.url : null,
+    variants: product.variants ?? [],
   };
 }