Improve bootstrap handling in SenderEditorPage

Added a `DxLoadingPanel` to display a loading indicator during the bootstrap process. Introduced the `IsBootstrapLoading` property to track bootstrap progress. Refactored `OnParametersSetAsync` to handle bootstrap tokens more effectively, including redirecting to `/sender` if no token is provided. Updated `TryApplyBootstrapAsync` to adjust navigation behavior when the bootstrap payload is `null`.

Enhanced `OnAfterRenderAsync` to process `_pendingBootstrapToken` more cleanly and removed redundant logic. Simplified navigation and improved code readability and maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/SenderEditorPage.razor

@@ -31,6 +31,11 @@
 <link href="@AppVersion.GetVersionedUrl("css/envelope-viewer.css")" rel="stylesheet" />
 <script src="@AppVersion.GetVersionedUrl("js/envelope-editor.js")"></script>
 
+<DxLoadingPanel Visible="@IsBootstrapLoading"
+                IsContentBlocked="true"
+                ApplyBackgroundShading="true"
+                IndicatorAreaVisible="true"
+                Text='@L("SenderEditor_BootstrapLoadingHint", "Der Umschlag wird vorbereitet und die Daten werden uebernommen.")'>
 <div class="envelope-viewer-layout">
 
     @* ── Action Bar ── *@
@@ -426,6 +431,7 @@
         }
     </div>
 </div>
+</DxLoadingPanel>
 
 <DxPopup @bind-Visible="_receiverPopupVisible"
          HeaderText='@L("AddReceiver", "Empfänger hinzufügen")'
@@ -670,6 +676,7 @@
     static readonly System.ComponentModel.DataAnnotations.EmailAddressAttribute ReceiverEmailValidator = new();
 
     bool IsEditMode => EnvelopeId.HasValue && EnvelopeId.Value > 0;
+    bool IsBootstrapLoading => !IsEditMode && !_bootstrapApplied;
 
     // ── Lifecycle ──
 
@@ -704,17 +711,22 @@
         if (EnvelopeId.HasValue && EnvelopeId.Value > 0)
             _editingEnvelopeId = EnvelopeId.Value;
 
-        if (!IsEditMode && !_bootstrapApplied && !string.IsNullOrWhiteSpace(BootstrapToken))
-            _pendingBootstrapToken = BootstrapToken;
-
         if (IsEditMode && _loadedEnvelopeId != EnvelopeId)
         {
             await LoadEnvelopeForEditAsync(EnvelopeId!.Value);
             return;
         }
 
-        if (IsEditMode)
+        if (IsEditMode || _bootstrapApplied)
             return;
+
+        if (string.IsNullOrWhiteSpace(BootstrapToken))
+        {
+            NavigationManager.NavigateTo("/sender", replace: true);
+            return;
+        }
+
+        _pendingBootstrapToken = BootstrapToken;
     }
 
     async Task TryApplyBootstrapAsync(string token)
@@ -724,7 +736,7 @@
 
         if (payload is null)
         {
-            NavigationManager.NavigateTo("/sender/editor", replace: true);
+            NavigationManager.NavigateTo("/sender", replace: true);
             return;
         }
 
@@ -785,8 +797,6 @@
             _pagePreviews = [];
             _pageCount = 0;
         }
-
-        NavigationManager.NavigateTo("/sender/editor", replace: true);
     }
 
     protected override async Task OnAfterRenderAsync(bool firstRender)
@@ -800,6 +810,15 @@
         if (!OperatingSystem.IsBrowser())
             return;
 
+        if (!IsEditMode && !_bootstrapApplied && !string.IsNullOrWhiteSpace(_pendingBootstrapToken))
+        {
+            var token = _pendingBootstrapToken;
+            _pendingBootstrapToken = null;
+            await TryApplyBootstrapAsync(token);
+            await InvokeAsync(StateHasChanged);
+            return;
+        }
+
         if (_saveDraftAfterBootstrap && !_isSaving)
         {
             _saveDraftAfterBootstrap = false;
@@ -807,12 +826,6 @@
             return;
         }
 
-        if (!IsEditMode && !_bootstrapApplied && !string.IsNullOrWhiteSpace(_pendingBootstrapToken))
-        {
-            var token = _pendingBootstrapToken;
-            _pendingBootstrapToken = null;
-            await TryApplyBootstrapAsync(token);
-        }
     }
 
     async Task LoadEnvelopeForEditAsync(int envelopeId)