PR U-3: validate every row in TransactionSplitDialog

The single EditContext only wrapped the first split row; subsequent
rows added by AddTransactionAsync silently bypassed
DataAnnotationsValidator. Replace editContext.Validate() with an
explicit ValidateAllRows() loop that checks Text, TargetAccountId, and
Value != 0 for every row, and render the collected messages under the
existing FluentValidationSummary.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Schaad.Accounting.UI/Components/Pages/Dialogs/TransactionSplitDialog.razor

@@ -73,6 +73,15 @@
         </table>
         <div>
             <FluentValidationSummary/>
+            @if (validationErrors.Count > 0)
+            {
+                <ul class="validation-message">
+                    @foreach (var err in validationErrors)
+                    {
+                        <li>@err</li>
+                    }
+                </ul>
+            }
         </div>
     </EditForm>
 </FluentDialogBody>

Schaad.Accounting.UI/Components/Pages/Dialogs/TransactionSplitDialog.razor.cs

@@ -36,6 +36,7 @@ public partial class TransactionSplitDialog : ComponentBase
     private IReadOnlyList<AccountDataset> accounts = [];
     private IReadOnlyList<string> bookingTexts = [];
     private decimal openAmount = 0;
+    private List<string> validationErrors = new();
 
 
     protected override async Task OnInitializedAsync()
@@ -68,16 +69,43 @@ public partial class TransactionSplitDialog : ComponentBase
         openAmount = Content.Value + transactionList.Sum(t => t.Value);
     }
 
+    // Every row in transactionList needs Text, TargetAccountId, and a non-zero Value.
+    // The single EditContext above only covers the first row (that's what it was
+    // constructed with) — so DataAnnotations validation misses subsequent split rows.
+    // Run explicit per-row checks here and surface everything in validationErrors.
+    private bool ValidateAllRows()
+    {
+        validationErrors.Clear();
+        for (var i = 0; i < transactionList.Count; i++)
+        {
+            var t = transactionList[i];
+            if (string.IsNullOrWhiteSpace(t.Text))
+            {
+                validationErrors.Add($"Zeile {i + 1}: Buchungstext fehlt");
+            }
+            if (string.IsNullOrEmpty(t.TargetAccountId))
+            {
+                validationErrors.Add($"Zeile {i + 1}: Konto fehlt");
+            }
+            if (t.Value == 0)
+            {
+                validationErrors.Add($"Zeile {i + 1}: Betrag darf nicht 0 sein");
+            }
+        }
+        return validationErrors.Count == 0;
+    }
+
     private async Task SaveAsync()
     {
-        if (editContext.Validate())
+        if (!ValidateAllRows())
         {
-            foreach (var transtaction in transactionList)
+            return;
-            {
-                await transactionRepository.SaveTransactionAsync(transtaction);
-            }
-            await Dialog.CloseAsync(Content);
         }
+        foreach (var transtaction in transactionList)
+        {
+            await transactionRepository.SaveTransactionAsync(transtaction);
+        }
+        await Dialog.CloseAsync(Content);
     }
 
     private async Task CancelAsync()