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/wwwroot/css/envelope-viewer.css

@@ -61,13 +61,34 @@
     padding: 1rem;
 }
 
+.sender-image-editor-shell {
+    position: relative;
+    width: 100%;
+    height: 100%;
+}
+
+.sender-image-editor__toolbar-host {
+    position: sticky;
+    top: 0;
+    z-index: 40;
+    height: 0;
+    pointer-events: none;
+}
+
 .sender-image-toolbar {
+    pointer-events: auto;
+    position: absolute;
+    top: 0.75rem;
+    left: 50%;
+    transform: translateX(-50%);
+    width: calc(100% - 2rem);
     max-width: 980px;
-    margin: 0 auto 1rem;
+    margin: 0;
     display: grid;
     grid-template-columns: minmax(0, 1.4fr) minmax(0, 1fr) auto;
     gap: 0.75rem;
     align-items: center;
+    background: transparent;
 }
 
 .sender-image-toolbar__group {
@@ -141,11 +162,12 @@
     display: flex;
     flex-direction: column;
     gap: 1.25rem;
+    padding-top: 6.2rem;
 }
 
 .sender-image-editor__native-preview {
     max-width: 980px;
-    margin: 0 auto 1rem;
+    margin: 6.2rem auto 1rem;
     background: #ffffff;
     border: 1px solid #cbd5e1;
     border-radius: 10px;
@@ -1250,12 +1272,22 @@ body.resizing {
 
 .sender-image-toolbar {
     grid-template-columns: 1fr;
+    width: calc(100% - 1rem);
+    top: 0.5rem;
 }
 
 .sender-image-toolbar__group {
     min-height: 3.9rem;
 }
 
+.sender-image-editor__pages {
+    padding-top: 12.2rem;
+}
+
+.sender-image-editor__native-preview {
+    margin-top: 12.2rem;
+}
+
 .sender-image-toolbar__value-box {
     min-width: 7.5rem;
     font-size: 1rem;