feat: enhance CADCanvas to support block creation from selected elements

frontend/src/components/CADCanvas.jsx

@@ -43,6 +43,37 @@ const CADCanvas = forwardRef(({ activeTool, setActiveTool, onCanvasReady }, ref)
         if (shape) fabricRef.current.add(shape);
       });
       fabricRef.current.renderAll();
+    },
+    // Get selected elements for block creation
+    getSelectedElements: () => {
+      if (!fabricRef.current) return [];
+      const activeObject = fabricRef.current.getActiveObject();
+      if (!activeObject) return [];
+      
+      // If it's a group, return all objects in the group
+      if (activeObject.type === 'group') {
+        return activeObject.getObjects().map(obj => ({
+          id: obj.id || Date.now() + Math.random(),
+          type: obj.type,
+          ...obj.toObject(['id', 'type', 'left', 'top', 'width', 'height', 'radius', 'x1', 'y1', 'x2', 'y2', 'points', 'fill', 'stroke', 'strokeWidth'])
+        }));
+      }
+      
+      // If it's a single object, return it
+      return [{
+        id: activeObject.id || Date.now() + Math.random(),
+        type: activeObject.type,
+        ...activeObject.toObject(['id', 'type', 'left', 'top', 'width', 'height', 'radius', 'x1', 'y1', 'x2', 'y2', 'points', 'fill', 'stroke', 'strokeWidth'])
+      }];
+    },
+    // Generate SVG data for selected elements
+    generateSvgData: () => {
+      if (!fabricRef.current) return '';
+      const activeObject = fabricRef.current.getActiveObject();
+      if (!activeObject) return '';
+      
+      // Clone the object to avoid modifying the original
+      return activeObject.toSVG();
     }
   }));