Add PdfFileDropZone for interactive PDF uploads

Replaced static upload message in `CreateEnvelopeWizardPopup.razor` with a new `PdfFileDropZone` component to enhance user experience. The component supports drag-and-drop functionality, file selection, and dynamic states (e.g., disabled, drag-active).

Introduced `PdfFileDropZone.razor` with customizable parameters such as `Disabled`, `HasDocument`, `UploadText`, and `DragDropHintText`. Added event handling for drag-and-drop interactions and file selection.

Updated `sender-page.css` to include styles for the new component, ensuring seamless integration with the existing layout. Adjusted styles for hover effects, drag-active state, and disabled state.

These changes improve usability and make PDF uploads more intuitive.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/sender-page.css

@@ -1466,6 +1466,66 @@
     display: none;
 }
 
+.sender-pdf-dropzone {
+    position: relative;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 92px;
+    border: 1px dashed rgba(100, 116, 139, 0.5);
+    border-radius: 8px;
+    background: rgba(248, 250, 252, 0.9);
+    color: #334155;
+    cursor: pointer;
+    transition: border-color 120ms ease, background-color 120ms ease, box-shadow 120ms ease;
+}
+
+.sender-pdf-dropzone:hover {
+    border-color: rgba(126, 34, 206, 0.45);
+    background: rgba(241, 245, 249, 0.95);
+}
+
+.sender-pdf-dropzone--drag-active {
+    border-color: rgba(16, 185, 129, 0.75);
+    background: linear-gradient(180deg, rgba(16, 185, 129, 0.14), rgba(16, 185, 129, 0.08));
+    box-shadow: 0 0 0 3px rgba(16, 185, 129, 0.16);
+}
+
+.sender-pdf-dropzone--disabled {
+    opacity: 0.55;
+    cursor: not-allowed;
+}
+
+.sender-pdf-dropzone__content {
+    text-align: center;
+    display: grid;
+    gap: 0.2rem;
+    padding: 0.75rem 0.9rem;
+}
+
+.sender-pdf-dropzone__title {
+    font-size: 0.86rem;
+    font-weight: 700;
+}
+
+.sender-pdf-dropzone__hint {
+    font-size: 0.78rem;
+    color: #64748b;
+}
+
+.sender-pdf-dropzone__input {
+    position: absolute;
+    inset: 0;
+    width: 100%;
+    height: 100%;
+    opacity: 0;
+    cursor: pointer;
+}
+
+.sender-pdf-dropzone--disabled .sender-pdf-dropzone__input {
+    cursor: not-allowed;
+}
+
 .sender-create-wizard__document-meta {
     border: 1px solid rgba(16, 185, 129, 0.3);
     border-radius: 8px;
@@ -1524,6 +1584,10 @@
         width: 100%;
     }
 
+    .sender-pdf-dropzone {
+        width: 100%;
+    }
+
     .sender-create-wizard__upload-btn {
         width: 100%;
     }