s&box: All inventory/weapon mutations must run on host — use Rpc.Host wrappers for client calls

In s&box multiplayer, inventory operations like Pickup, Drop, SwitchWeapon, MoveSlot, and Remove must only execute on the host. If a client calls these methods directly, they silently do nothing (the guard if (!Networking.IsHost) return false exits early). Developers often forget to add the host-side RPC wrapper, leaving clients unable to interact with their own inventory.