fix: Add missing antiforgery token to Messages Reply form

- Added @Html.AntiForgeryToken() to Customer.cshtml reply form
- Fixes HTTP 400 error when replying to customer messages
- Required for CSRF protection with [ValidateAntiForgeryToken]

The form was missing the CSRF token which caused ASP.NET Core to reject
all POST requests to /Admin/Messages/Reply with HTTP 400 status.

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

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

LittleShop/Areas/Admin/Views/Messages/Customer.cshtml

@@ -143,6 +143,7 @@
                         </div>
                         <div class="card-body">
                             <form method="post" action="@Url.Action("Reply")">
+                                @Html.AntiForgeryToken()
                                 <input type="hidden" name="customerId" value="@Model.CustomerId" />
                                 
                                 <div class="mb-3">