feat(shop): add box pickup/carry, pour-into-shelf, and package flight visuals

- Carryable: generic pickup/carry component, cosmetic per-peer follow of
  the carrier's HandSocket instead of NGO reparenting (camera pitch isn't
  networked, so root-level reparenting wouldn't tilt with the view)
- PlayerCarry: mutual exclusion with PlayerHands, server-timed hold-to-pour
  tick (client only signals start/stop, never per-unit requests)
- IPourSource/IPourTarget: decouple PlayerInteractor from concrete Shop
  types, same pattern as IInteractable
- Lieferkarton: gains product data (productId/quantity), two-stage
  Interact() (open, then pick up), IPourSource
- ShelfSlot: implements IPourTarget, fills the previous RefreshVisuals TODO
- PackageDisplay: renders quantity as a proper column x row grid that
  stacks into further layers as space runs out, reused by both containers
- FlyingPackage: small non-networked cosmetic visual that arcs a package
  from the carried box to its exact landing slot in the shelf grid on
  every successful pour tick, broadcast via Rpc(SendTo.Everyone)
- StorageCrate: rejects pickup while the player is carrying a box

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

Assets/Scripts/Shop/ShelfSlot.cs

@@ -10,7 +10,7 @@ namespace AfterHours.Shop
     /// Ein Regalfach: hält genau eine Produktsorte. Auffüllen geht über
     /// Interact(), Entnahme durch Kunden serverseitig über TakeOne().
     /// </summary>
-    public sealed class ShelfSlot : NetworkBehaviour, IInteractable
+    public sealed class ShelfSlot : NetworkBehaviour, IInteractable, IPourTarget
     {
         [SerializeField] private ProductCatalog _catalog;
         [SerializeField] private int _capacityUnits = 12;
@@ -20,6 +20,11 @@ namespace AfterHours.Shop
 
         [SerializeField] private Transform _displayRoot;
 
+        [Header("Anzeige-Raster (sauber in Reihen eingeräumt)")]
+        [SerializeField] private int _displayColumns = 4;
+        [SerializeField] private int _displayRows = 3;
+        [SerializeField] private Vector3 _displaySpacing = new(0.1f, 0.1f, 0.1f);
+
         private readonly NetworkVariable<int> _productId = new(-1);
         private readonly NetworkVariable<int> _quantity = new(0);
         private readonly NetworkVariable<float> _price = new(0f);
@@ -29,6 +34,20 @@ namespace AfterHours.Shop
         public float Price => _price.Value;
         public bool ExposedToStreet => _exposedToStreet;
         public bool IsEmpty => _quantity.Value <= 0;
+        public bool IsFull => _quantity.Value >= Capacity();
+
+        // Explizite Implementierung, weil "transform" (Component) den Namen schon belegt.
+        Transform IPourTarget.Transform => transform;
+
+        private PackageDisplay _packageDisplay;
+
+        private void Awake() => _packageDisplay = new PackageDisplay(_displayRoot, _displayColumns, _displayRows, _displaySpacing);
+
+        /// <summary>Weltposition des Rasterplatzes mit gegebenem Index – Ziel fürs Flug-Visual (siehe PlayerCarry).</summary>
+        public Vector3 GetSlotWorldPosition(int index)
+        {
+            return _displayRoot != null ? _displayRoot.TransformPoint(_packageDisplay.SlotLocalPosition(index)) : transform.position;
+        }
 
         public override void OnNetworkSpawn()
         {
@@ -99,6 +118,24 @@ namespace AfterHours.Shop
             Debug.Log($"[ShelfSlot] Eingeräumt: {moved} Stück, jetzt {_quantity.Value}/{Capacity()}.");
         }
 
+        /// <summary>Server: ein Stück aus einem IPourSource einräumen (siehe PlayerCarry).</summary>
+        public bool TryAddOne(int productId)
+        {
+            if (!IsServer) return false;
+            if (!IsEmpty && _productId.Value != productId) return false;
+            if (_quantity.Value >= Capacity()) return false;
+
+            if (IsEmpty)
+            {
+                _productId.Value = productId;
+                var def = _catalog.Get(productId);
+                if (def != null && _price.Value <= 0f) _price.Value = def.RecommendedPrice;
+            }
+
+            _quantity.Value++;
+            return true;
+        }
+
         /// <summary>Server: Kunde entnimmt ein Stück.</summary>
         public bool TakeOne()
         {
@@ -126,8 +163,9 @@ namespace AfterHours.Shop
 
         private void RefreshVisuals()
         {
-            // TODO: Verpackungs-Prefabs nach Menge ein-/ausblenden (Object Pool).
-            //       Läuft auf allen Peers, rein kosmetisch.
+            var product = _catalog != null ? _catalog.Get(_productId.Value) : null;
+            GameObject prefab = product != null ? product.PackagePrefab : null;
+            _packageDisplay.SetTarget(prefab, _quantity.Value, Capacity());
         }
     }
 }