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.

src/CanvasView.ts

@@ -815,6 +815,8 @@ $el("label.clipboard-switch.mask-switch", {
         }
     }, [canvas.canvas]) as HTMLDivElement;
 
+    canvas.canvasContainer = canvasContainer;
+
     const layersPanelContainer = $el("div.painterLayersPanelContainer", {
         style: {
             position: "absolute",