Replaces direct usage of document.createElement('canvas') and manual context setup with the createCanvas utility across multiple utility modules. This change improves code consistency, reduces duplication, and centralizes canvas/context creation logic. Also updates notification usage in ClipboardManager to use showNotification and showInfoNotification utilities.
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.
