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/Areas/Admin/Views/Shared/_Layout.cshtml

@@ -66,6 +66,11 @@
                                 <i class="fas fa-box"></i> Products
                             </a>
                         </li>
+                        <li class="nav-item">
+                            <a class="nav-link" href="@Url.Action("Index", "VariantCollections", new { area = "Admin" })">
+                                <i class="fas fa-layer-group"></i> Variants
+                            </a>
+                        </li>
                         <li class="nav-item">
                             <a class="nav-link" href="@Url.Action("Index", "Orders", new { area = "Admin" })">
                                 <i class="fas fa-shopping-cart"></i> Orders
@@ -208,6 +213,12 @@
                     Products
                 </a>
             </li>
+            <li class="settings-menu-item">
+                <a href="@Url.Action("Index", "VariantCollections", new { area = "Admin" })" class="settings-menu-link">
+                    <i class="fas fa-layer-group"></i>
+                    Variant Collections
+                </a>
+            </li>
             <li class="settings-menu-item">
                 <a href="@Url.Action("Index", "ShippingRates", new { area = "Admin" })" class="settings-menu-link">
                     <i class="fas fa-truck"></i>