Fix three minor ShelfBlock issues

[okcoder123]

A few small QoL/cleanup ShelfBlock fixes:

• swapHotbar checks the hotbar slot with a bounds check of <= inventory.getContainerSize() which should be < because otherwise it is off by one. ATM it isn't reachable because the slot stays under the container size, but could cause hazards in future if not handled.
• Replaced the unchecked (ShelfBlockEntity) cast and the null check in the same loop with an instanceof pattern match
• useItemOn ignored the off hand which blocked proper handling of offhand use of the Shelf like in vanilla, so changed to match and have vanilla parity.