feat(web-client-tpos): connect all remaining admin pages to real backend APIs
- BFF: Added 10 new endpoints (staff roles/schedules, orders, wallets, devices, promotions, inventory transactions, membership levels)
- PosDataService: Added 14 new client methods with DTOs
- Rewrote 19 admin pages from hardcoded to real API:
Staff: Create, Schedule, Attendance, Payroll
Finance: Overview, Revenue, Expenses, Tax
Inventory: PurchaseOrders, StockTransfer, SupplierMgmt
Product: MenuBuilder, ModifierGroups, PricingRules
Customer: Feedback, LoyaltyProgram
System: DeviceManagement, NotificationCenter, IntegrationHub
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