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/DTOs/ProductDto.cs

@@ -14,6 +14,8 @@ public class ProductDto
     public int StockQuantity { get; set; }
     public Guid CategoryId { get; set; }
     public string CategoryName { get; set; } = string.Empty;
+    public Guid? VariantCollectionId { get; set; }
+    public string? VariantsJson { get; set; }
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }
     public bool IsActive { get; set; }
@@ -36,46 +38,54 @@ public class CreateProductDto
     [Required]
     [StringLength(200)]
     public string Name { get; set; } = string.Empty;
-    
+
     [Required(AllowEmptyStrings = true)]
     public string Description { get; set; } = string.Empty;
-    
+
     [Required]
     [Range(0.01, double.MaxValue, ErrorMessage = "Price must be greater than 0")]
     public decimal Price { get; set; }
-    
+
     [Required]
     [Range(0.01, double.MaxValue, ErrorMessage = "Weight must be greater than 0")]
     public decimal Weight { get; set; }
-    
+
     public ProductWeightUnit WeightUnit { get; set; } = ProductWeightUnit.Grams;
-    
+
     [Range(0, int.MaxValue)]
     public int StockQuantity { get; set; } = 0;
-    
+
     [Required(ErrorMessage = "Please select a category")]
     public Guid CategoryId { get; set; }
+
+    public Guid? VariantCollectionId { get; set; }
+
+    public string? VariantsJson { get; set; }
 }
 
 public class UpdateProductDto
 {
     [StringLength(200)]
     public string? Name { get; set; }
-    
+
     public string? Description { get; set; }
-    
+
     [Range(0.01, double.MaxValue)]
     public decimal? Price { get; set; }
-    
+
     public decimal? Weight { get; set; }
-    
+
     public ProductWeightUnit? WeightUnit { get; set; }
-    
+
     [Range(0, int.MaxValue)]
     public int? StockQuantity { get; set; }
-    
+
     public Guid? CategoryId { get; set; }
-    
+
+    public Guid? VariantCollectionId { get; set; }
+
+    public string? VariantsJson { get; set; }
+
     public bool? IsActive { get; set; }
 }