Implement robust world-based positioning for Blend Mode menu

Redesigned the positioning system for the Blend Mode menu, inspired by the "Custom Output Area" logic. The menu now anchors precisely to the top-right corner of the viewport and stays in place during panning and zooming.

js/CanvasView.js

@@ -762,6 +762,7 @@ async function createCanvasWidget(node, widget, app) {
             overflow: "hidden"
         }
     }, [canvas.canvas]);
+    canvas.canvasContainer = canvasContainer;
     const layersPanelContainer = $el("div.painterLayersPanelContainer", {
         style: {
             position: "absolute",