feat: add ExtendTool implementation

frontend/src/tools/modification/ExtendTool.ts

@@ -0,0 +1,18 @@
+import { Tool } from '../Tool';
+import { Canvas } from '../../canvas/Canvas';
+
+export class ExtendTool extends Tool {
+  name = 'Extend';
+  icon = 'extend';
+  
+  constructor(canvas: Canvas) {
+    super(canvas);
+  }
+  
+  async execute(): Promise<void> {
+    // Implementation for extend tool
+    // Select boundary and elements to extend
+    // Update Yjs document with extended elements
+    // Add to undo history
+  }
+}