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/types.ts

@@ -16,6 +16,7 @@ export interface Layer {
     zIndex: number;
     blendMode: string;
     opacity: number;
+    visible: boolean;
     mask?: Float32Array;
     flipH?: boolean;
     flipV?: boolean;