Mark already-purchased items on the wishlist instead of auto-removing

A customer often wishlists something specifically to buy it again (gifts, repurchases) — silently removing it after purchase would defeat that. /konto/merkliste now shows a dimmed image + "Gekauft am [date]" badge instead, derived read-only from the customer's own orders (cancelled/returned orders excluded). Removal stays manual.

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

README.md

@@ -2313,6 +2313,29 @@ features requires the matching `CompanySettings` toggle to actually be
 switched on before it shows up anywhere in the frontend — if a feature
 "isn't showing," check that first.
 
+## Wishlist "already purchased" marker (2026-07-31)
+
+Decided against auto-removing a wishlist item once the customer buys it —
+a customer often wishlists something specifically *to* buy it again
+(gifts for multiple people, anything they'd repurchase), so silently
+removing it right when they'd next want it defeats the point. Instead,
+`/konto/merkliste` (`MerklisteGrid.tsx`) marks it: dimmed image + a
+"Gekauft am [date]" badge (replaces the "Ausverkauft" badge when both
+would apply — already-owning it matters more than a restock notice).
+Removal stays entirely manual, same `WishlistButton` as always.
+
+- **`WishlistItem.purchasedAt`** (`app/lib/customerAuth.ts`) — never
+  persisted on the backend's `wishlist-items` collection (its `update`
+  access is hard-disabled by design); derived fresh on every
+  `getWishlist()` call by cross-referencing the customer's own orders
+  (`getPurchasedVariantMap()`), matched on the exact `(product, variant)`
+  pair a wishlist row represents. Excludes `cancelled`/`returned` orders
+  — an aborted or refunded order doesn't mean the customer actually owns
+  the item.
+- Flows through `useWishlist.ts`'s existing item shape/cache/broadcast
+  mechanism unchanged — this is purely an added read-only field, not a
+  new toggle or a second collection.
+
 ## DHL checkout integrations (2026-07-31)
 
 Three checkout-facing pieces added, each invisible unless the tenant's