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/invoicePdf.tsx

@@ -85,7 +85,7 @@ function formatPrice(amount: number): string {
   return new Intl.NumberFormat("de-DE", { style: "currency", currency: "EUR" }).format(amount);
 }
 
-export type InvoiceItem = { productName: string; quantity: number; unitPrice: number; taxRatePercent: number; bundleContents?: string | null };
+export type InvoiceItem = { productName: string; quantity: number; unitPrice: number; taxRatePercent: number; bundleContents?: string | null; variantName?: string | null };
 
 export type InvoiceOrder = {
   orderNumber: string;
@@ -266,7 +266,10 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
             {order.items.map((item, i) => (
               <View style={[styles.tableRow, i % 2 === 1 ? styles.tableRowAlt : {}]} key={i}>
                 <View style={styles.colName}>
-                  <Text>{item.productName}</Text>
+                  <Text>
+                    {item.productName}
+                    {item.variantName ? ` (${item.variantName})` : ""}
+                  </Text>
                   {item.bundleContents ? <Text style={styles.bundleLine}>{item.bundleContents}</Text> : null}
                 </View>
                 <Text style={styles.colQty}>{item.quantity}</Text>