Add layer copy/paste and node duplication with layers

Implements two new features:
- Layer copy/paste within canvas using Ctrl+C/V
- Node duplication that preserves all layers

Layer Copy/Paste:
- Added Ctrl+V keyboard shortcut handler for pasting layers
- Intercept keydown events during capture phase to handle before ComfyUI
- Focus canvas when layer is clicked to ensure shortcuts work
- Prevent layers panel from stealing focus on mousedown

Node Duplication:
- Store source node ID during serialize for copy operations
- Track pending copy sources across node ID changes (-1 to real ID)
- Copy canvas state from source to destination in onAdded hook
- Use Map to persist copy metadata through node lifecycle

js/CanvasInteractions.js

@@ -104,6 +104,8 @@ export class CanvasInteractions {
         // Add a blur event listener to the window to reset key states
         window.addEventListener('blur', this.onBlur);
         document.addEventListener('paste', this.onPaste);
+        // Intercept Ctrl+V during capture phase to handle layer paste before ComfyUI
+        document.addEventListener('keydown', this.onKeyDown, { capture: true });
         this.canvas.canvas.addEventListener('mouseenter', this.onMouseEnter);
         this.canvas.canvas.addEventListener('mouseleave', this.onMouseLeave);
         this.canvas.canvas.addEventListener('dragover', this.onDragOver);
@@ -119,6 +121,8 @@ export class CanvasInteractions {
         this.canvas.canvas.removeEventListener('wheel', this.onWheel);
         this.canvas.canvas.removeEventListener('keydown', this.onKeyDown);
         this.canvas.canvas.removeEventListener('keyup', this.onKeyUp);
+        // Remove document-level capture listener
+        document.removeEventListener('keydown', this.onKeyDown, { capture: true });
         window.removeEventListener('blur', this.onBlur);
         document.removeEventListener('paste', this.onPaste);
         this.canvas.canvas.removeEventListener('mouseenter', this.onMouseEnter);
@@ -570,6 +574,12 @@ export class CanvasInteractions {
                         this.canvas.canvasLayers.copySelectedLayers();
                     }
                     break;
+                case 'v':
+                    // Paste layers from internal clipboard
+                    if (this.canvas.canvasLayers.internalClipboard.length > 0) {
+                        this.canvas.canvasLayers.pasteLayers();
+                    }
+                    break;
                 default:
                     handled = false;
                     break;