feat(shop): configurable package display rotation, pocoregal shelf model

Add _displayRotationEuler to Lieferkarton/ShelfSlot so package prefabs
with a non-upright export orientation can be corrected per-container
without touching PackageDisplay code. Fixes a bug where flying packages
landed with a different rotation than the resting shelf display
(PlayerCarry used the raw shelf transform instead of
ShelfSlot.GetSlotWorldRotation()).

Also adds the pocoregal shelf model (Blender export) and its materials,
places it in the Shop scene, registers ShelfSlot as a network prefab,
and rescales EmptyBox. See docs/PACKAGE_DISPLAY_SETUP.md for the
display-grid setup/troubleshooting reference.

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

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