Add batch preview manager and mask overlay toggle

Introduces BatchPreviewManager for reviewing and confirming multiple imported layers after auto-refresh. Adds a toggle button for mask overlay visibility in the UI and updates mask rendering logic to respect overlay visibility. Also refactors image import to return new layers and adds a utility for removing layers by ID.

js/CanvasLayers.js

@@ -178,6 +178,7 @@ export class CanvasLayers {
         }
 
         const layer = {
+            id: generateUUID(),
             image: image,
             imageId: imageId,
             x: finalX,