Add export feature UI to EnvelopeSenderPage (placeholder)

Introduced a new export feature to the EnvelopeSenderPage, including a dropdown for selecting export formats (XLSX, CSV, PDF) and a checkbox to export only selected rows. Added a placeholder export button ("Exportieren") that is currently disabled, with export logic to be implemented in a future phase.

Added private fields `_selectedExportFormat` and `_exportSelectedRowsOnly` to manage the export state. Implemented `IsExportDisabled()` to control button availability and a placeholder method `ExportSelectedFormatPlaceholderAsync()`.

Updated `sender-page.css` to style the export feature, including the export button, dropdown, and checkbox. Added theme-specific styles for "purple" and "blazing-dark" themes. Adjusted toolbar element order to position the export group before the logout button.

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

@@ -104,6 +104,31 @@
                     Vorschau
                 </button>
 
+                <div class="sender-export-group" title="Export der aktuellen Tabellenansicht">
+                    <label class="sender-export-label" for="sender-export-format">Format</label>
+                    <select id="sender-export-format" class="sender-export-format" @bind="_selectedExportFormat" aria-label="Exportformat auswählen">
+                        <option value="xlsx">XLSX</option>
+                        <option value="csv">CSV</option>
+                        <option value="pdf">PDF</option>
+                    </select>
+
+                    <label class="sender-export-selected">
+                        <input type="checkbox" @bind="_exportSelectedRowsOnly" />
+                        <span>Nur ausgewählte</span>
+                    </label>
+
+                    <button class="sender-btn sender-btn--export"
+                            @onclick="ExportSelectedFormatPlaceholderAsync"
+                            disabled="@IsExportDisabled()"
+                            title="Export wird in Phase 2 aktiviert">
+                        <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
+                    </button>
+                </div>
+
                 <button class="sender-btn sender-btn--logout" @onclick="LogoutAsync" disabled="@_isLoggingOut" title="Abmelden">
                     @if (_isLoggingOut)
                     {
@@ -514,6 +539,10 @@
     private byte[]? _previewPdfBytes = null;
     private string? _previewErrorMessage = null;
 
+    // Phase 6e (step 1): UI state only. Export logic is added in step 2.
+    private string _selectedExportFormat = "xlsx";
+    private bool _exportSelectedRowsOnly = false;
+
     // Stable keys used to store/restore each grid's layout in UserPreferences.GridLayouts
     private const string GridKeyActive = "sender.active-envelopes";
     private const string GridKeyCompleted = "sender.completed-envelopes";
@@ -572,6 +601,19 @@
         await LoadEnvelopesAsync();
     }
 
+    bool IsExportDisabled()
+    {
+        if (_isLoading)
+            return true;
+
+        return _exportSelectedRowsOnly && _selectedEnvelope is null;
+    }
+
+    Task ExportSelectedFormatPlaceholderAsync()
+    {
+        return Task.CompletedTask;
+    }
+
     void CreateEnvelope()
     {
         Navigation.NavigateTo("/sender/editor");
@@ -753,4 +795,4 @@
         var json = JsonSerializer.Serialize(e.Layout);
         await UserPreferencesService.SaveGridLayoutAsync(GridKeyCompleted, json);
     }
-}
+}