Add drag-and-drop PDF support to Blazor app

Implement drag-and-drop PDF loading via JS interop and DotNetObjectReference. Refactor file loading logic and UI structure for clarity. Add IAsyncDisposable for resource cleanup. Update pdfInterop.js to handle drop events and send PDF data to Blazor.

EnvelopeGenerator.ReceiverUIBlazor/wwwroot/js/pdfInterop.js

@@ -264,6 +264,39 @@
 
             pointerPads.set(canvasId, { ctx, canvas });
         },
+        registerDropHandler: (dotNetRef) => {
+            if (window.__pdfDropRegistered) return;
+            window.__pdfDropRegistered = true;
+
+            const prevent = (e) => {
+                e.preventDefault();
+                e.stopPropagation();
+            };
+
+            ['dragenter', 'dragover', 'dragleave'].forEach(evt => {
+                document.addEventListener(evt, prevent, false);
+            });
+
+            document.addEventListener('drop', (e) => {
+                prevent(e);
+
+                const files = e.dataTransfer?.files;
+                if (!files || files.length === 0) {
+                    return;
+                }
+
+                const file = files[0];
+                const reader = new FileReader();
+                reader.onload = () => {
+                    const result = reader.result;
+                    if (typeof result === 'string') {
+                        const base64 = result.split(',')[1] || result;
+                        dotNetRef?.invokeMethodAsync('LoadPdfFromBase64', base64);
+                    }
+                };
+                reader.readAsDataURL(file);
+            }, false);
+        },
         clearSignaturePad: (canvasId) => {
             const pad = pointerPads.get(canvasId);
             if (!pad) return;