Fix: improve EditTransaction form handling and validation

- Add hidden fields for immutable transaction properties to preserve values during form submission
- Make category field optional by removing validation for empty values
- Simplify category input handling by removing duplicate hidden field
- Clean up JavaScript by using proper element IDs instead of querySelector

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

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

MoneyMap/Pages/EditTransaction.cshtml

@@ -34,6 +34,12 @@
             <div class="card-body">
                 <form method="post">
                     <input type="hidden" asp-for="Transaction.Id" />
+                    <input type="hidden" asp-for="Transaction.Date" />
+                    <input type="hidden" asp-for="Transaction.Name" />
+                    <input type="hidden" asp-for="Transaction.Memo" />
+                    <input type="hidden" asp-for="Transaction.Amount" />
+                    <input type="hidden" asp-for="Transaction.CardLabel" />
+                    <input type="hidden" asp-for="Transaction.AccountLabel" />
 
                     <!-- Read-only fields -->
                     <div class="row mb-3">
@@ -89,11 +95,10 @@
                         <div id="customCategoryInput" style="display: @(Model.UseCustomCategory ? "block" : "none")">
                             <input asp-for="Transaction.Category"
                                    class="form-control"
+                                   id="categoryInput"
                                    placeholder="Enter custom category" />
                         </div>
 
-                        <input type="hidden" asp-for="Transaction.Category" id="categoryHidden" />
-
                         <div class="form-text">Select a category or create a new one</div>
                         <span asp-validation-for="Transaction.Category" class="text-danger"></span>
                     </div>
@@ -261,16 +266,15 @@
     <script>
         function handleCategoryChange() {
             const select = document.getElementById('categorySelect');
-            const customInput = document.getElementById('customCategoryInput');
-            const hiddenInput = document.getElementById('categoryHidden');
-            const categoryInput = document.querySelector('input[name="Transaction.Category"]');
+            const customInputDiv = document.getElementById('customCategoryInput');
+            const categoryInput = document.getElementById('categoryInput');
 
             if (select.value === '__custom__') {
-                customInput.style.display = 'block';
+                customInputDiv.style.display = 'block';
                 categoryInput.value = '';
                 categoryInput.focus();
             } else {
-                customInput.style.display = 'none';
+                customInputDiv.style.display = 'none';
                 categoryInput.value = select.value;
             }
         }

MoneyMap/Pages/EditTransaction.cshtml.cs

@@ -97,6 +97,12 @@ namespace MoneyMap.Pages
 
         public async Task<IActionResult> OnPostAsync()
         {
+            // Remove Category from model state validation if it's empty (category is optional)
+            if (string.IsNullOrWhiteSpace(Transaction.Category))
+            {
+                ModelState.Remove("Transaction.Category");
+            }
+
             // Remove Notes from model state validation if it's empty
             if (string.IsNullOrWhiteSpace(Transaction.Notes))
             {