Differentiate between multi-channel retailing and omnichannel retailing from a theoretical perspective. Focus on the necessity of a unified customer database that allows for seamless transitions between online storefronts, mobile apps, and physical brick-and-mortar locations. Discuss the 'customer-centric' approach where the channel is chosen by the customer, not the retailer. Analyze the challenges in achieving true channel synchronization in terms of inventory visibility, pricing consistency, and customer support.