Add layer visibility toggle and icon support

Introduces a 'visible' property to layers and updates all relevant logic to support toggling layer visibility. Adds visibility toggle icons to the layers panel using a new IconLoader utility, with SVG and fallback canvas icons. Updates rendering, selection, and batch preview logic to respect layer visibility. Also improves blend mode menu UI and ensures new/pasted layers are always added on top with correct z-index.

src/CanvasIO.ts

@@ -546,7 +546,8 @@ export class CanvasIO {
                 originalWidth: image.width,
                 originalHeight: image.height,
                 blendMode: 'normal',
-                opacity: 1
+                opacity: 1,
+                visible: true
             };
 
             this.canvas.layers.push(layer);
@@ -729,6 +730,7 @@ export class CanvasIO {
                 zIndex: this.canvas.layers.length,
                 blendMode: 'normal',
                 opacity: 1,
+                visible: true,
             };
 
             this.canvas.layers.push(layer);