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/Interaction/IPourTarget.cs

@@ -0,0 +1,19 @@
+using UnityEngine;
+
+namespace AfterHours.Interaction
+{
+    /// <summary>
+    /// Alles, was per Klick-halten Stück für Stück befüllt werden kann
+    /// (z. B. ShelfSlot, später Lagerregal). TryAddOne() läuft serverseitig
+    /// und muss selbst validieren, genau wie IInteractable.Interact().
+    /// </summary>
+    public interface IPourTarget
+    {
+        Transform Transform { get; }
+        int ProductId { get; }
+        int Quantity { get; }
+        bool IsEmpty { get; }
+        bool IsFull { get; }
+        bool TryAddOne(int productId);
+    }
+}