Add inventory management features including Sales, Stock Returns, and Suppliers

- Implement Sale and SaleItem models for tracking sales transactions.
- Create StockReturn model to manage stock adjustments for various reasons.
- Introduce Supplier model to represent suppliers of products.
- Develop InventoryRepository for CRUD operations on products, suppliers, sales, and stock returns.
- Implement InventoryService to handle business logic for inventory management.
- Update TenantDbContext to include new models and relationships.
- Enhance ProfitLossRepository to account for sales, purchases, and stock returns in profit and loss calculations.
- Modify Program.cs to register new inventory-related services.

CentralDbContext.cs

@@ -152,6 +152,7 @@ namespace Fitness_core_BE
                 new NavItem { Key = "memberships", Label = "Memberships", Href = "/dashboard/memberships", Icon = "CreditCard", SortOrder = 3, IsOwnerOnly = false },
                 new NavItem { Key = "bookings", Label = "Bookings", Href = "/dashboard/bookings", Icon = "CalendarClock", SortOrder = 4, IsOwnerOnly = false },
                 new NavItem { Key = "customer-payments", Label = "Customer Payments", ShortLabel = "Payments", Href = "/dashboard/customer-payments", Icon = "Receipt", SortOrder = 5, IsOwnerOnly = false },
+                new NavItem { Key = "inventory", Label = "Inventory", Href = "/dashboard/inventory", Icon = "Package", SortOrder = 6, IsOwnerOnly = false },
                 new NavItem { Key = "workout-plans", Label = "Workout Plans", Href = "/dashboard/workout-plans", Icon = "Activity", SortOrder = 7, IsOwnerOnly = false },
                 new NavItem { Key = "meal-plans", Label = "Meal Plans", Href = "/dashboard/meal-plans", Icon = "Utensils", SortOrder = 8, IsOwnerOnly = false },
                 new NavItem { Key = "staff", Label = "Staff", Href = "/dashboard/staff", Icon = "UsersRound", SortOrder = 9, IsOwnerOnly = false },