Add mask editor integration to canvas workflow

Introduces the ability to open the current canvas in a mask editor, upload and retrieve mask edits, and apply them to the mask layer. Adds utility functions for mask editor state detection and control, a new 'Edit Mask' button in the UI, and methods for handling mask updates and preview refresh. Also adds a setMask method to MaskTool for precise mask placement.

js/CanvasView.js

@@ -761,6 +761,13 @@ async function createCanvasWidget(node, widget, app) {
             ]),
             $el("div.painter-separator"),
             $el("div.painter-button-group", {id: "mask-controls"}, [
+                $el("button.painter-button", {
+                    textContent: "Edit Mask",
+                    title: "Open the current canvas view in the mask editor",
+                    onclick: () => {
+                        canvas.startMaskEditor();
+                    }
+                }),
                 $el("button.painter-button", {
                     id: "mask-mode-btn",
                     textContent: "Draw Mask",