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>

TeleBot/TeleBot/UI/MenuBuilder.cs

@@ -376,11 +376,44 @@ namespace TeleBot.UI
                 var variantButtons = new List<InlineKeyboardButton>();
                 foreach (var variant in group.OrderBy(v => v.SortOrder))
                 {
+                    string variantInfo = "";
+
+                    if (variant.StockLevel > 0)
+                    {
+                        variantInfo = $" ({variant.StockLevel} in stock";
+                    }
+                    else if (variant.StockLevel == 0)
+                    {
+                        variantInfo = " (Out of stock";
+                    }
+
+                    if (variant.Weight.HasValue)
+                    {
+                        var unitName = variant.WeightUnit switch
+                        {
+                            1 => "µg",
+                            2 => "g",
+                            3 => "oz",
+                            4 => "lb",
+                            5 => "ml",
+                            6 => "L",
+                            _ => "unit"
+                        };
+                        variantInfo += variantInfo == "" ? $" ({variant.Weight}{unitName}" : $", {variant.Weight}{unitName}";
+                    }
+
+                    if (variantInfo != "")
+                    {
+                        variantInfo += ")";
+                    }
+
                     // For multi-buy, allow multiple selections
                     if (quantity > 1)
                     {
                         var count = selectedVariants.Count(v => v == variant.Name);
-                        var buttonText = count > 0 ? $"{variant.Name} ({count})" : variant.Name;
+                        var buttonText = count > 0
+                            ? $"{variant.Name} ({count}){variantInfo}"
+                            : $"{variant.Name}{variantInfo}";
 
                         variantButtons.Add(InlineKeyboardButton.WithCallbackData(
                             buttonText,
@@ -391,7 +424,9 @@ namespace TeleBot.UI
                     {
                         // Single item, select one variant
                         var isSelected = selectedVariants.Contains(variant.Name);
-                        var buttonText = isSelected ? $"✅ {variant.Name}" : variant.Name;
+                        var buttonText = isSelected
+                            ? $"✅ {variant.Name}{variantInfo}"
+                            : $"{variant.Name}{variantInfo}";
 
                         variantButtons.Add(InlineKeyboardButton.WithCallbackData(
                             buttonText,