Add resizable thumbnail sidebar to EnvelopeViewer

Introduced a resizable splitter for the PDF thumbnail sidebar, allowing users to dynamically adjust its width. Added `_thumbnailWidth` property with min/max constraints and implemented mouse event handlers (`OnSplitterMouseDown`, `OnSplitterMouseMove`, `OnSplitterMouseUp`) to manage resizing.

Integrated JavaScript interop to attach/detach resize event listeners and save user preferences to `localStorage`. Updated `pdf-viewer.js` to handle resizing state and cleanup. Styled the splitter in `envelope-viewer.css` with hover/active states and ensured smooth interaction.

Persisted thumbnail width across sessions and added error handling for `localStorage`. Enhanced user experience with intuitive resizing and improved UI flexibility.

EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css

@@ -104,6 +104,46 @@
     background: linear-gradient(135deg, #6b1cb0 0%, #1e3a72 100%);
 }
 
+.pdf-splitter {
+    width: 4px;
+    background: transparent;
+    cursor: col-resize;
+    flex-shrink: 0;
+    position: relative;
+    transition: background 0.2s ease;
+    z-index: 10;
+    user-select: none;
+}
+
+.pdf-splitter::before {
+    content: '';
+    position: absolute;
+    left: -4px;
+    right: -4px;
+    top: 0;
+    bottom: 0;
+    /* Enlarged hitbox for easier grabbing */
+}
+
+.pdf-splitter:hover,
+.pdf-splitter.resizing {
+    background: linear-gradient(90deg, 
+        rgba(126, 34, 206, 0.4) 0%, 
+        rgba(42, 82, 152, 0.4) 100%);
+}
+
+.pdf-splitter:active {
+    background: linear-gradient(90deg, 
+        rgba(126, 34, 206, 0.6) 0%, 
+        rgba(42, 82, 152, 0.6) 100%);
+}
+
+/* Prevent text selection during resize */
+body.resizing {
+    user-select: none;
+    cursor: col-resize !important;
+}
+
 .pdf-thumbnail {
     cursor: pointer;
     border-radius: 8px;