Add brush preview overlay to MaskTool

Introduces a brush preview overlay using a separate preview canvas in MaskTool. Mouse event handlers in CanvasInteractions and MaskTool are updated to support passing both world and view coordinates, enabling accurate brush preview rendering. The preview is shown or hidden appropriately on mouse enter/leave and while drawing.

js/Canvas.js

@@ -247,6 +247,20 @@ export class Canvas {
         return {x: worldX, y: worldY};
     }
 
+    getMouseViewCoordinates(e) {
+        const rect = this.canvas.getBoundingClientRect();
+        const mouseX_DOM = e.clientX - rect.left;
+        const mouseY_DOM = e.clientY - rect.top;
+
+        const scaleX = this.canvas.width / rect.width;
+        const scaleY = this.canvas.height / rect.height;
+
+        const mouseX_Canvas = mouseX_DOM * scaleX;
+        const mouseY_Canvas = mouseY_DOM * scaleY;
+
+        return { x: mouseX_Canvas, y: mouseY_Canvas };
+    }
+
 
     moveLayer(fromIndex, toIndex) {
         return this.canvasLayers.moveLayer(fromIndex, toIndex);