feat: implement server-side cart persistence with Stripe PaymentIntents
Replaced the client-only in-mem cart with a database-backed cart system
- Add Cart and CartItem GORM models (two-table design)
- Add 6 cart API endpoints (get, set item, set address, set card, clear,
conver)
- Switch from Stripe Charge API to PaymentIntents with confirm:true
- Add 9 API client methods for cart and card operations
- Refactor TUI cart from map[int] (slice index) to map[uint] (CoffeeID)
- Replace single-shot checkout with saveCardAndConvert multi-step flow
- Add CheckingOut state with "submitting order..." processing indicator
- Fix Stripe tokenization to use secret key for server-side SSH apps
- Remove admin product CRUD endpoints and old checkout handler
