fix(files): report success cancellation and errors with safe next actions (U02)

src/Duempelkas.App/Pages/Accounts/AccountDetail.razor

@@ -7,6 +7,9 @@
 @inject NavigationManager Navigation
 
 <div class="container-fluid">
+    <OperationNotice Result="fileResult" />
+    @if (fileBusy) { <p role="status" aria-live="polite">Dateiaktion läuft … Bitte warten.</p> }
+
     @if (account == null)
     {
         <div class="text-center py-5">
@@ -35,7 +38,7 @@
             <button class="btn btn-info btn-nav" @onclick="() => showAddTransfer = true">
                 <i class="bi bi-arrow-left-right"></i> Umbuchung
             </button>
-            <button class="btn btn-dark btn-nav" @onclick="HandleExport">
+            <button class="btn btn-dark btn-nav" @onclick="HandleExport" disabled="@fileBusy">
                 <i class="bi bi-file-earmark-pdf"></i> PDF
             </button>
             |

src/Duempelkas.App/Pages/Accounts/AccountDetail.razor.cs

@@ -32,6 +32,8 @@ public partial class AccountDetail
     private string? confirmRestoreEntryTitle;
 
     private string? savedPdfPath;
+    private FileOperationResult? fileResult;
+    private bool fileBusy;
 
     private EntryDto? editingEntry;
     private EntryDto? editingTransferEntry;
@@ -178,19 +180,39 @@ public partial class AccountDetail
 
     private async Task HandleExport()
     {
+        if (fileBusy) return;
+        fileBusy = true;
+        fileResult = null;
+        try
+        {
         var pdf = await PdfStatementService.GenerateStatementAsync(AccountId, selectedYear);
         var suffix = selectedYear.HasValue ? $"_{selectedYear.Value}" : "_Gesamt";
-        savedPdfPath = await FileSaveService.SaveFileAsync(pdf, $"{account?.Name}{suffix}.pdf");
+        fileResult = await FileSaveService.SaveFileAsync(pdf, $"{account?.Name}{suffix}.pdf");
+
+            savedPdfPath = fileResult.Status == FileOperationStatus.Success ? fileResult.Path : null;
+
+        }
+        catch (Exception ex) { fileResult = FileOperationResult.Error(ex); }
+        finally { fileBusy = false; }
     }
 
     private async Task HandleOpenSavedPdf()
     {
+        if (fileBusy) return;
+        fileBusy = true;
+        fileResult = null;
+        try
+        {
         if (!string.IsNullOrWhiteSpace(savedPdfPath))
         {
-            await FileSaveService.OpenFileAsync(savedPdfPath);
+            fileResult = await FileSaveService.OpenFileAsync(savedPdfPath);
         }
 
         savedPdfPath = null;
+
+        }
+        catch (Exception ex) { fileResult = FileOperationResult.Error(ex); }
+        finally { fileBusy = false; }
     }
 
     private void CancelOpenSavedPdf()