Fix: Add missing CSRF token to Categories Edit form

**Issue**: Edit category functionality failing with AntiforgeryValidationException
- Error: "The required antiforgery request token was not provided"
- POST requests to /Admin/Categories/Edit/{id} returning 400 Bad Request

**Root Cause**:
- Categories/Edit.cshtml form missing @Html.AntiForgeryToken()
- Create and Delete forms already had the token
- Edit was the only form missing CSRF protection

**Solution**:
- Added @Html.AntiForgeryToken() to Edit.cshtml (line 19)
- Matches pattern used in Create.cshtml and Index.cshtml delete forms

**Files Changed**:
- LittleShop/Areas/Admin/Views/Categories/Edit.cshtml

**Testing**:
- Deployed to production (container: littleshop-admin restarted)
- Edit category form now includes __RequestVerificationToken field

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

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

LittleShop/Areas/Admin/Views/Categories/Edit.cshtml

@@ -16,6 +16,7 @@
         <div class="card">
             <div class="card-body">
                 <form method="post" action="@Url.Action("Edit", new { id = categoryId })">
+                    @Html.AntiForgeryToken()
                     @if (ViewData.ModelState.ErrorCount > 0)
                     {
                         <div class="alert alert-danger" role="alert">