Improve document upload UX and refactor wizard layout

Enhanced the `CreateEnvelopeWizardPopup` component:
- Added a new grid layout (`sender-create-wizard__body-layout`) to separate main content and side panel.
- Introduced a collapsible side panel for document upload and preview.
- Replaced old upload logic with `PdfFileDropZone` and added support for displaying document metadata (name, size) and replacement hints.
- Added asynchronous document preview rendering with `EnsureDocumentPreviewPreparedAsync`.

Localization updates:
- Added new localized strings for document upload hints in `de-DE`, `en-US`, and `fr-FR`.

CSS updates:
- Styled the new layout and side panel with transitions.
- Updated `PdfFileDropZone` styles for different states (e.g., document uploaded).

Refactored code for better maintainability and improved user feedback during document upload and preview preparation.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/Sender/PdfFileDropZone.razor

@@ -2,10 +2,24 @@
 
 <div class="sender-pdf-dropzone @DropZoneClass">
     <div class="sender-pdf-dropzone__content">
-        <div class="sender-pdf-dropzone__title">@CurrentActionText</div>
-        @if (!string.IsNullOrWhiteSpace(DragDropHintText))
+        @if (HasDocument)
         {
-            <div class="sender-pdf-dropzone__hint">@DragDropHintText</div>
+            @if (!string.IsNullOrWhiteSpace(DocumentName))
+            {
+                <div class="sender-pdf-dropzone__doc-name">@DocumentName</div>
+            }
+            @if (!string.IsNullOrWhiteSpace(DocumentSizeText))
+            {
+                <div class="sender-pdf-dropzone__doc-size">@DocumentSizeText</div>
+            }
+            @if (!string.IsNullOrWhiteSpace(UploadedHintText))
+            {
+                <div class="sender-pdf-dropzone__hint">@UploadedHintText</div>
+            }
+        }
+        else if (!string.IsNullOrWhiteSpace(CurrentActionText))
+        {
+            <div class="sender-pdf-dropzone__hint">@CurrentActionText</div>
         }
     </div>
 
@@ -38,6 +52,15 @@
     [Parameter]
     public string DragDropHintText { get; set; } = "Datei hierhin ziehen oder klicken";
 
+    [Parameter]
+    public string? DocumentName { get; set; }
+
+    [Parameter]
+    public string? DocumentSizeText { get; set; }
+
+    [Parameter]
+    public string UploadedHintText { get; set; } = "Zum Ersetzen klicken oder eine Datei hierher ziehen.";
+
     [Parameter]
     public EventCallback<InputFileChangeEventArgs> FileSelected { get; set; }
 
@@ -53,6 +76,9 @@
             if (Disabled)
                 return "sender-pdf-dropzone--disabled";
 
+            if (HasDocument)
+                return "sender-pdf-dropzone--has-document";
+
             return _isDragActive ? "sender-pdf-dropzone--drag-active" : string.Empty;
         }
     }