feat: implement PDF preview popup on sender dashboard

Replace the placeholder preview popup with a fully functional DxPdfViewer
popup:

- Popup resized to 80vw × 90vh with drag, resize and background shading.
- OpenPreviewAsync: unmounts viewer, fetches PDF bytes via
  GetDocumentOfSenderAsync, then mounts DxPdfViewer only after bytes are
  ready (prevents DevExpress ProcessDocumentChangedAsync NRE).
- Loading spinner shown while fetching; typed error panel shown on
  NotFoundException or general failure.
- OnPreviewClosed: unmounts viewer before nulling byte array so DevExpress
  async render never touches a disposed reference.
- Inject ReceiverPageDataService, EnvelopeAuthService and ILogger.

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

@@ -4,13 +4,18 @@
 @using EnvelopeGenerator.Domain.Constants
 @using EnvelopeGenerator.Server.Client.Models
 @using DevExpress.Blazor
+@using DevExpress.Blazor.PdfViewer
 @using EnvelopeGenerator.Server.Client.Services
 @using EnvelopeGenerator.Application.Common.Dto
+@using EnvelopeGenerator.Server.Services
 @inject EnvelopeGenerator.Server.Client.Services.EnvelopeService EnvelopeService
 @inject EnvelopeGenerator.Server.Client.Services.AuthService AuthService
 @inject EnvelopeGenerator.Server.Client.Services.ThemeService ThemeService
 @inject EnvelopeGenerator.Server.Client.Services.UserPreferencesService UserPreferencesService
 @inject EnvelopeGenerator.Server.Client.Services.CustomImagesService CustomImagesService
+@inject EnvelopeReceiverPageDataService ReceiverPageDataService
+@inject IEnvelopeAuthService EnvelopeAuthService
+@inject ILogger<EnvelopeSenderPage> Logger
 @inject NavigationManager Navigation
 @inject IJSRuntime JSRuntime
 @inject AppVersionService AppVersion
@@ -428,22 +433,63 @@
 
 @* ── PDF Vorschau Popup ────────────────────────────────────────────────── *@
 <DxPopup @bind-Visible="@_previewVisible"
-         HeaderText="@($"Vorschau — Umschlag #{_selectedEnvelope?.Id}")"
-         Width="480px"
+         HeaderText="@($"Vorschau — {_selectedEnvelope?.Title} (#{_selectedEnvelope?.Id})")"
+         Width="80vw"
+         MaxWidth="80vw"
+         Height="90vh"
+         MaxHeight="90vh"
          CloseOnOutsideClick="true"
-         ShowFooter="true">
+         ShowFooter="false"
+         CssClass="sender-preview-popup"
+         Closed="OnPreviewClosed"
+         AllowResize="true"
+         ApplyBackgroundShading="true"
+         Scrollable="true"
+         AllowDrag=true
+         EnableAdaptivity=true
+         ShowHeader=true>
     <BodyContentTemplate>
-        <div style="padding: 1rem;">
-            <p><strong>Umschlag-ID:</strong> @_selectedEnvelope?.Id</p>
-            <p><strong>Titel:</strong> @_selectedEnvelope?.Title</p>
-        </div>
+        @if (_previewLoading)
+        {
+            <div style="display:flex;justify-content:center;align-items:center;height:100%;min-height:300px;">
+                <div class="text-center">
+                    <div class="spinner-border mb-3" style="width:3rem;height:3rem;" role="status">
+                        <span class="visually-hidden">Lädt...</span>
+                    </div>
+                    <p style="color:#6b7280;font-size:0.875rem;">Dokument wird geladen…</p>
+                </div>
+            </div>
+        }
+        else if (_previewErrorMessage is not null)
+        {
+            <div style="display:flex;gap:1rem;align-items:flex-start;padding:1.5rem;">
+                <div style="flex-shrink:0;width:40px;height:40px;border-radius:50%;
+                            background:#fee2e2;display:flex;align-items:center;justify-content:center;">
+                    <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" fill="#dc2626" viewBox="0 0 16 16">
+                        <path d="M8 15A7 7 0 1 1 8 1a7 7 0 0 1 0 14zm0 1A8 8 0 1 0 8 0a8 8 0 0 0 0 16z" />
+                        <path d="M7.002 11a1 1 0 1 1 2 0 1 1 0 0 1-2 0zM7.1 4.995a.905.905 0 1 1 1.8 0l-.35 3.507a.552.552 0 0 1-1.1 0L7.1 4.995z" />
+                    </svg>
+                </div>
+                <div>
+                    <p style="margin:0 0 0.4rem;font-weight:600;color:#1f2937;font-size:0.95rem;">
+                        Dokument konnte nicht geladen werden
+                    </p>
+                    <p style="margin:0;color:#6b7280;font-size:0.85rem;line-height:1.5;word-break:break-word;">
+                        @_previewErrorMessage
+                    </p>
+                </div>
+            </div>
+        }
+        else if (_previewPdfBytes is not null && _previewViewerMounted)
+        {
+            <div class="sender-preview-viewer-wrap" style="padding: 0 5.5rem;">
+                <DxPdfViewer DocumentContent="@_previewPdfBytes"
+                             ZoomLevel="-2"
+                             IsSinglePagePreview="true"
+                             CssClass="sender-preview-pdf-viewer" />
+            </div>
+        }
     </BodyContentTemplate>
-    <FooterContentTemplate>
-        <DxButton CssClass="popup-button my-1 ms-2"
-                  RenderStyle="ButtonRenderStyle.Secondary"
-                  Text="Schließen"
-                  Click="@(() => _previewVisible = false)" />
-    </FooterContentTemplate>
 </DxPopup>
 
 @code {
@@ -464,6 +510,10 @@
 
     // Preview popup
     private bool _previewVisible = false;
+    private bool _previewLoading = false;
+    private bool _previewViewerMounted = false;
+    private byte[]? _previewPdfBytes = null;
+    private string? _previewErrorMessage = null;
 
     // Stable keys used to store/restore each grid's layout in UserPreferences.GridLayouts
     private const string GridKeyActive = "sender.active-envelopes";
@@ -599,13 +649,69 @@
         var clicked = e.Grid.GetDataItem(e.VisibleIndex) as EnvelopeDto;
         if (clicked == null) return;
         _selectedEnvelope = clicked;
-        _previewVisible = true;
+        _ = OpenPreviewAsync(clicked);
     }
 
-    void ShowPreview()
+    async Task ShowPreview()
     {
         if (_selectedEnvelope == null) return;
+        await OpenPreviewAsync(_selectedEnvelope);
+    }
+
+    private async Task OpenPreviewAsync(EnvelopeDto envelope)
+    {
+        // Unmount the viewer first — let the previous instance fully dispose
+        _previewViewerMounted = false;
+        _previewPdfBytes = null;
+        _previewErrorMessage = null;
+        _previewLoading = true;
         _previewVisible = true;
+        await InvokeAsync(StateHasChanged);
+
+        try
+        {
+            var user = EnvelopeAuthService.GetCurrentUser();
+            if (user?.Identity is null || !user.Identity.IsAuthenticated)
+            {
+                _previewErrorMessage = "Nicht authentifiziert. Bitte melden Sie sich erneut an.";
+                return;
+            }
+
+            _previewPdfBytes = await ReceiverPageDataService.GetDocumentOfSenderAsync(user, envelope.Id);
+
+            // PDF ready — mount the viewer now
+            _previewViewerMounted = true;
+        }
+        catch (DigitalData.Core.Exceptions.NotFoundException)
+        {
+            _previewErrorMessage = $"Das Dokument für Umschlag #{envelope.Id} wurde nicht gefunden oder Sie haben keinen Zugriff darauf.";
+            Logger.LogWarning("Preview: document not found for envelope {EnvelopeId}", envelope.Id);
+        }
+        catch (Exception ex)
+        {
+            _previewErrorMessage = $"Fehler beim Laden des Dokuments: {ex.Message}";
+            Logger.LogError(ex, "Preview: failed to load document for envelope {EnvelopeId}", envelope.Id);
+        }
+        finally
+        {
+            _previewLoading = false;
+            await InvokeAsync(StateHasChanged);
+        }
+    }
+
+    void OnPreviewClosed()
+    {
+        // Unmount the viewer first — prevent DevExpress async render from
+        // accessing the already-disposed byte array
+        _previewViewerMounted = false;
+        _ = InvokeAsync(async () =>
+        {
+            await Task.Yield(); // wait one render cycle for the viewer to be removed from the DOM
+            _previewPdfBytes = null;
+            _previewErrorMessage = null;
+            _previewLoading = false;
+            await InvokeAsync(StateHasChanged);
+        });
     }
 
     // ── Grid Layout Persistence ───────────────────────────────────────────────