added dedicated *Switch() methods (ShopSwitch, CartSwitc, AccountSwitch, ...) for page transitions and remove the catch-all resetPageState(). Each switch owns its pages zero-init, viewport setup, and async data fetching
prices stored as float64 could lead to rounding errors when converting to cents for stripe (19.99 * 100 = 1998.99... truncating to 1998). canged Coffee.Price to int, storing cents directly. Updated the seed data, fallback products, API handlers, and all TUI display code to be in sync with this change.
