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

src/Duempelkas.App/Pages/Dashboard.razor

@@ -8,6 +8,9 @@
 @inject NavigationManager NavigationManager
 
 <div class="container-fluid">
+    <OperationNotice Result="fileResult" />
+    @if (fileBusy) { <p role="status" aria-live="polite">Dateiaktion läuft … Bitte warten.</p> }
+
 
     <div class="d-flex justify-content-between align-items-center">
         <div class="d-flex align-items-center flex-grow-1" style="min-width: 0;">
@@ -25,14 +28,14 @@
             <i class="bi bi-plus-lg"></i> Neues Konto
         </button>
         |
-        <button class="btn-nav btn-success" @onclick="HandleBackupAsync">
+        <button class="btn-nav btn-success" @onclick="HandleBackupAsync" disabled="@fileBusy">
             <i class="bi bi-save"></i> Backup
         </button>
-        <button class="btn-nav btn-warning" @onclick="HandleRestoreAsync">
+        <button class="btn-nav btn-warning" @onclick="HandleRestoreAsync" disabled="@fileBusy">
             <i class="bi bi-arrow-counterclockwise"></i> Restore
         </button>
         |
-        <button class="btn-nav btn-dark" @onclick="HandleDashboardExportAsync">
+        <button class="btn-nav btn-dark" @onclick="HandleDashboardExportAsync" disabled="@fileBusy">
             <i class="bi bi-file-earmark-pdf"></i> PDF
         </button>
         |
@@ -54,12 +57,6 @@
         </div>
     }
 
-    @if (!string.IsNullOrWhiteSpace(operationMessage))
-    {
-        <div class="alert @operationMessageClass mb-3" role="alert">
-            @operationMessage
-        </div>
-    }
 
     @if (accounts == null)
     {