Add variant collections system and enhance ProductVariant with weight/stock tracking

This commit introduces a comprehensive variant management system and enhances
the existing ProductVariant model with per-variant weight overrides and stock
tracking, integrated across Admin Panel and TeleBot.

Features Added:
- Variant Collections: Reusable variant templates (e.g., "Standard Sizes")
- Admin UI for managing variant collections (CRUD operations)
- Dynamic variant editor with JavaScript-based UI
- Per-variant weight and weight unit overrides
- Per-variant stock level tracking
- SalesLedger model for financial tracking

ProductVariant Enhancements:
- Added Weight (decimal, nullable) field for variant-specific weights
- Added WeightUnit (enum, nullable) field for variant-specific units
- Maintains backward compatibility with product-level weights

TeleBot Integration:
- Enhanced variant selection UI to display stock levels
- Shows weight information with proper unit conversion (µg, g, oz, lb, ml, L)
- Compact button format: "Medium (15 in stock, 350g)"
- Real-time stock availability display

Database Migrations:
- 20250928014850_AddVariantCollectionsAndSalesLedger
- 20250928155814_AddWeightToProductVariants

Technical Changes:
- Updated Product model to support VariantCollectionId and VariantsJson
- Extended ProductService with variant collection operations
- Enhanced OrderService to handle variant-specific pricing and weights
- Updated LittleShop.Client DTOs to match server models
- Added JavaScript dynamic variant form builder

Files Modified: 15
Files Added: 17
Lines Changed: ~2000

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop/Models/Product.cs

@@ -24,9 +24,13 @@ public class Product
     public ProductWeightUnit WeightUnit { get; set; } = ProductWeightUnit.Kilogram;
     
     public int StockQuantity { get; set; } = 0;
-    
+
     public Guid CategoryId { get; set; }
-    
+
+    public Guid? VariantCollectionId { get; set; }
+
+    public string? VariantsJson { get; set; }
+
     public bool IsActive { get; set; } = true;
     
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
@@ -35,9 +39,11 @@ public class Product
     
     // Navigation properties
     public virtual Category Category { get; set; } = null!;
+    public virtual VariantCollection? VariantCollection { get; set; }
     public virtual ICollection<ProductPhoto> Photos { get; set; } = new List<ProductPhoto>();
     public virtual ICollection<ProductMultiBuy> MultiBuys { get; set; } = new List<ProductMultiBuy>();
     public virtual ICollection<ProductVariant> Variants { get; set; } = new List<ProductVariant>();
+    public virtual ICollection<SalesLedger> SalesLedgers { get; set; } = new List<SalesLedger>();
     public virtual ICollection<BotActivity> Activities { get; set; } = new List<BotActivity>();
     public virtual ICollection<OrderItem> OrderItems { get; set; } = new List<OrderItem>();
     public virtual ICollection<Review> Reviews { get; set; } = new List<Review>();