Restore: Add Variants menu option back to navigation

Re-added Variants menu item to both desktop and mobile navigation.
User needs access to ProductVariants management to create variant collections.

Changes:
- Desktop nav: Added Variants between Products and Orders
- Mobile drawer: Added Variants between Products and Shipping

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

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

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

@@ -94,6 +94,11 @@
                                 <i class="fas fa-box"></i> Products
                             </a>
                         </li>
+                        <li class="nav-item">
+                            <a class="nav-link" href="@Url.Action("Index", "ProductVariants", 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
@@ -236,6 +241,12 @@
                     Products
                 </a>
             </li>
+            <li class="settings-menu-item">
+                <a href="@Url.Action("Index", "ProductVariants", new { area = "Admin" })" class="settings-menu-link">
+                    <i class="fas fa-layer-group"></i>
+                    Variants
+                </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>