Bedrock offhand support

First, in Touch Settings, add a toggle: "Swap to left hand with hotbar." When enabled, double-tapping a selected hotbar slot (e.g., slot 1) moves its item to the offhand. If the offhand already has an item, double-tapping will swap the hotbar item and offhand item. Next, add another toggle: "Left hand button." When enabled, a new on-screen button (like the pick block button) appears. Pressing it swaps the currently selected hotbar item with the offhand item. For PC, pressing F will swap the selected hotbar item with the offhand. Also include classic drag-and-drop: players can move items between inventory and offhand freely. Mechanism: Currently, shields require holding Shift and fireworks can’t be used from offhand, meaning there’s no right-click functionality for offhand. First, add right-click support for offhand. Then implement dual-wielding rules: Offhand cannot perform left-click (attack/destroy). If both hands hold usable (right-clickable) items, only the main hand is used. The game checks the main hand first; if it has a usable item, offhand is ignored. If the main hand has no usable item, the offhand item is used.