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/emailTemplates.ts

@@ -163,6 +163,7 @@ export type OrderConfirmationItem = {
   unitPrice: number;
   imageUrl?: string | null;
   bundleContents?: string | null;
+  variantName?: string | null;
   taxRatePercent: number;
 };
 export type OrderConfirmationData = {
@@ -208,7 +209,7 @@ export function renderOrderConfirmationHtml(template: EmailTemplateContent, orde
               : `<div style="width:44px;height:44px;border-radius:6px;background:${BG_MUTED};"></div>`
           }
         </td>
-        <td style="padding:10px 0 10px 12px;border-bottom:1px solid ${BORDER};font-size:14px;color:${TEXT_PRIMARY};">${escapeHtml(item.productName)} <span style="color:${TEXT_MUTED};">× ${item.quantity}</span>${item.bundleContents ? `<br/><span style="font-size:12px;color:${TEXT_MUTED};">${escapeHtml(item.bundleContents)}</span>` : ""}</td>
+        <td style="padding:10px 0 10px 12px;border-bottom:1px solid ${BORDER};font-size:14px;color:${TEXT_PRIMARY};">${escapeHtml(item.productName)}${item.variantName ? ` (${escapeHtml(item.variantName)})` : ""} <span style="color:${TEXT_MUTED};">× ${item.quantity}</span>${item.bundleContents ? `<br/><span style="font-size:12px;color:${TEXT_MUTED};">${escapeHtml(item.bundleContents)}</span>` : ""}</td>
         <td style="padding:10px 0;border-bottom:1px solid ${BORDER};text-align:right;white-space:nowrap;font-size:14px;color:${TEXT_PRIMARY};">${formatPrice(item.quantity * item.unitPrice)}</td>
       </tr>`,
     )