feat: add MirrorTool implementation

frontend/src/tools/modification/MirrorTool.ts

@@ -0,0 +1,18 @@
+import { Tool } from '../Tool';
+import { Canvas } from '../../canvas/Canvas';
+
+export class MirrorTool extends Tool {
+  name = 'Mirror';
+  icon = 'mirror';
+  
+  constructor(canvas: Canvas) {
+    super(canvas);
+  }
+  
+  async execute(): Promise<void> {
+    // Implementation for mirror tool
+    // Select elements, get mirror axis
+    // Update Yjs document with mirrored positions
+    // Add to undo history
+  }
+}