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/Services/ProductService.cs

@@ -131,6 +131,8 @@ public class ProductService : IProductService
             StockQuantity = product.StockQuantity,
             CategoryId = product.CategoryId,
             CategoryName = product.Category.Name,
+            VariantCollectionId = product.VariantCollectionId,
+            VariantsJson = product.VariantsJson,
             CreatedAt = product.CreatedAt,
             UpdatedAt = product.UpdatedAt,
             IsActive = product.IsActive,
@@ -171,6 +173,8 @@ public class ProductService : IProductService
             WeightUnit = createProductDto.WeightUnit,
             StockQuantity = createProductDto.StockQuantity,
             CategoryId = createProductDto.CategoryId,
+            VariantCollectionId = createProductDto.VariantCollectionId,
+            VariantsJson = createProductDto.VariantsJson,
             CreatedAt = DateTime.UtcNow,
             UpdatedAt = DateTime.UtcNow,
             IsActive = true
@@ -226,6 +230,12 @@ public class ProductService : IProductService
         if (updateProductDto.CategoryId.HasValue)
             product.CategoryId = updateProductDto.CategoryId.Value;
 
+        if (updateProductDto.VariantCollectionId.HasValue)
+            product.VariantCollectionId = updateProductDto.VariantCollectionId.Value;
+
+        if (updateProductDto.VariantsJson != null)
+            product.VariantsJson = updateProductDto.VariantsJson;
+
         if (updateProductDto.IsActive.HasValue)
             product.IsActive = updateProductDto.IsActive.Value;