Implement export functionality with loading spinner

Replaced placeholder export method with a fully implemented
`ExportSelectedFormatAsync` method to support exporting grid
data in CSV, PDF, or XLSX formats. Added a loading spinner
to the export button to indicate progress during export
operations. Updated the `IsExportDisabled` method to account
for the new `_isExporting` state.

Disabled exporting for "Status" and "EnvelopeReceivers"
columns by setting their `ExportEnabled` property to `false`.
Made minor grid layout adjustments, including adding the
`KeyFieldName` property for proper grid behavior.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor

@@ -118,14 +118,21 @@
                     </label>
 
                     <button class="sender-btn sender-btn--export"
-                            @onclick="ExportSelectedFormatPlaceholderAsync"
+                            @onclick="ExportSelectedFormatAsync"
                             disabled="@IsExportDisabled()"
-                            title="Export wird in Phase 2 aktiviert">
+                            title="Aktuelle Ansicht exportieren">
                         <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16">
                             <path d="M.5 9.9a.5.5 0 0 1 .5.5V13a1 1 0 0 0 1 1h12a1 1 0 0 0 1-1v-2.6a.5.5 0 0 1 1 0V13a2 2 0 0 1-2 2H2a2 2 0 0 1-2-2v-2.6a.5.5 0 0 1 .5-.5z" />
                             <path d="M5.354 6.146a.5.5 0 0 1 .707 0L7.5 7.586V.5a.5.5 0 0 1 1 0v7.086l1.439-1.44a.5.5 0 0 1 .707.708l-2.293 2.293a.5.5 0 0 1-.707 0L5.354 6.854a.5.5 0 0 1 0-.708z" />
                         </svg>
-                        Exportieren
+                        @if (_isExporting)
+                        {
+                            <span class="spinner-border spinner-border-sm" style="width: 14px; height: 14px;"></span>
+                        }
+                        else
+                        {
+                            <span>Exportieren</span>
+                        }
                     </button>
                 </div>
 
@@ -209,6 +216,7 @@
                     {
                         <DxGrid Data="@_activeEnvelopes"
                                 @ref="_gridActive"
+                                KeyFieldName="Id"
                                 ShowFilterRow="true"
                                 ShowSearchBox="true"
                                 AllowColumnReorder="true"
@@ -238,7 +246,7 @@
                                         <strong>@((cellContext.DataItem as EnvelopeDto)?.Title)</strong>
                                     </CellDisplayTemplate>
                                 </DxGridDataColumn>
-                                <DxGridDataColumn FieldName="Status" Caption="Status">
+                                <DxGridDataColumn FieldName="Status" Caption="Status" ExportEnabled="false">
                                     <CellDisplayTemplate Context="cellContext">
                                         @{
                                             var envelope = cellContext.DataItem as EnvelopeDto;
@@ -253,7 +261,7 @@
                                         }
                                     </CellDisplayTemplate>
                                 </DxGridDataColumn>
-                                <DxGridDataColumn FieldName="EnvelopeReceivers" Caption="Empfänger">
+                                <DxGridDataColumn FieldName="EnvelopeReceivers" Caption="Empfänger" ExportEnabled="false">
                                     <CellDisplayTemplate Context="cellContext">
                                         @{
                                             var envelope = cellContext.DataItem as EnvelopeDto;
@@ -333,6 +341,7 @@
                     {
                         <DxGrid Data="@_completedEnvelopes"
                                 @ref="_gridCompleted"
+                                KeyFieldName="Id"
                                 ShowFilterRow="true"
                                 ShowSearchBox="true"
                                 PageSize="20"
@@ -359,7 +368,7 @@
                                         <strong>@((cellContext.DataItem as EnvelopeDto)?.Title)</strong>
                                     </CellDisplayTemplate>
                                 </DxGridDataColumn>
-                                <DxGridDataColumn FieldName="Status" Caption="Status">
+                                <DxGridDataColumn FieldName="Status" Caption="Status" ExportEnabled="false">
                                     <CellDisplayTemplate Context="cellContext">
                                         @{
                                             var envelope = cellContext.DataItem as EnvelopeDto;
@@ -374,7 +383,7 @@
                                         }
                                     </CellDisplayTemplate>
                                 </DxGridDataColumn>
-                                <DxGridDataColumn FieldName="EnvelopeReceivers" Caption="Empfänger">
+                                <DxGridDataColumn FieldName="EnvelopeReceivers" Caption="Empfänger" ExportEnabled="false">
                                     <CellDisplayTemplate Context="cellContext">
                                         @{
                                             var envelope = cellContext.DataItem as EnvelopeDto;
@@ -542,6 +551,7 @@
     // Phase 6e (step 1): UI state only. Export logic is added in step 2.
     private string _selectedExportFormat = "xlsx";
     private bool _exportSelectedRowsOnly = false;
+    private bool _isExporting = false;
 
     // Stable keys used to store/restore each grid's layout in UserPreferences.GridLayouts
     private const string GridKeyActive = "sender.active-envelopes";
@@ -603,15 +613,62 @@
 
     bool IsExportDisabled()
     {
-        if (_isLoading)
+        if (_isLoading || _isExporting)
             return true;
 
         return _exportSelectedRowsOnly && _selectedEnvelope is null;
     }
 
-    Task ExportSelectedFormatPlaceholderAsync()
+    async Task ExportSelectedFormatAsync()
     {
-        return Task.CompletedTask;
+        var grid = _activeTab == "active" ? _gridActive : _gridCompleted;
+        if (grid is null)
+            return;
+
+        _isExporting = true;
+        await InvokeAsync(StateHasChanged);
+
+        try
+        {
+            var tabToken = _activeTab == "completed" ? "abgeschlossen" : "aktiv";
+            var stamp = DateTime.Now.ToString("yyyy-MM-dd");
+            var exportBaseName = $"Umschlaege_{tabToken}_{stamp}";
+
+            switch (_selectedExportFormat)
+            {
+                case "csv":
+                    await grid.ExportToCsvAsync(
+                        exportBaseName,
+                        new GridCsvExportOptions
+                        {
+                            ExportSelectedRowsOnly = _exportSelectedRowsOnly
+                        });
+                    break;
+                case "pdf":
+                    await grid.ExportToPdfAsync(
+                        exportBaseName,
+                        new GridPdfExportOptions
+                        {
+                            ExportSelectedRowsOnly = _exportSelectedRowsOnly,
+                            SelectedRowsExportMode = GridSelectedRowsExportMode.KeepGrouping
+                        });
+                    break;
+                default:
+                    await grid.ExportToXlsxAsync(
+                        exportBaseName,
+                        new GridXlExportOptions
+                        {
+                            ExportSelectedRowsOnly = _exportSelectedRowsOnly,
+                            SelectedRowsExportMode = GridSelectedRowsExportMode.KeepGrouping
+                        });
+                    break;
+            }
+        }
+        finally
+        {
+            _isExporting = false;
+            await InvokeAsync(StateHasChanged);
+        }
     }
 
     void CreateEnvelope()