Refactor SenderPdfImageEditor layout and styles

Refactored `SenderPdfImageEditor.razor` to improve readability and maintainability by introducing a new wrapper element (`sender-image-editor-shell`) and reorganizing the toolbar into a sticky container (`sender-image-editor__toolbar-host`). Updated conditional rendering logic to handle `PageCount` cases more effectively.

Enhanced styles in `envelope-viewer.css`:
- Added `.sender-image-editor-shell` for layout structure.
- Made toolbar sticky with `.sender-image-editor__toolbar-host`.
- Improved alignment, spacing, and responsiveness of elements.
- Adjusted padding and margins for better layout consistency.

These changes improve user experience and maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/PdfEditor/SenderPdfImageEditor.razor

@@ -5,25 +5,28 @@
 @inject IStringLocalizer<Resource> UiLocalizer
 @inject IJSRuntime JSRuntime
 
-@if (PageCount > 0)
-{
-    <SenderPdfImageEditorToolbar CurrentPage="1"
-                                TotalPages="@PageCount"
-                                ZoomPercent="112" />
-}
-
-<div class="pdf-editor-wrapper sender-image-editor sender-editor-pdf-viewer"
-     style="cursor: @(IsPlacementMode ? "crosshair" : (_isDragging ? "grabbing" : "default"));"
-     @onclick="HandleClickAsync"
-     @onmousemove="OnEditorMouseMoveAsync"
-     @onmouseup="OnEditorMouseUp"
-     @onmouseleave="OnEditorMouseUp">
-    @if (PageCount <= 0)
+<div class="sender-image-editor-shell">
+    @if (PageCount > 0)
     {
-        <div class="sender-image-editor__empty">@L("SenderEditor_NoPagesToDisplay", "Keine Seiten zur Anzeige verfügbar.")</div>
+        <div class="sender-image-editor__toolbar-host">
+            <SenderPdfImageEditorToolbar CurrentPage="1"
+                                        TotalPages="@PageCount"
+                                        ZoomPercent="112" />
+        </div>
     }
-    else
-    {
+
+    <div class="pdf-editor-wrapper sender-image-editor sender-editor-pdf-viewer"
+         style="cursor: @(IsPlacementMode ? "crosshair" : (_isDragging ? "grabbing" : "default"));"
+         @onclick="HandleClickAsync"
+         @onmousemove="OnEditorMouseMoveAsync"
+         @onmouseup="OnEditorMouseUp"
+         @onmouseleave="OnEditorMouseUp">
+        @if (PageCount <= 0)
+        {
+            <div class="sender-image-editor__empty">@L("SenderEditor_NoPagesToDisplay", "Keine Seiten zur Anzeige verfügbar.")</div>
+        }
+        else
+        {
         @if (Pages.Count == 0 && !string.IsNullOrWhiteSpace(PdfDataUrl))
         {
             <div class="sender-image-editor__native-preview">
@@ -85,7 +88,8 @@
                 </div>
             }
         </div>
-    }
+        }
+    </div>
 </div>
 
 @code {