Act as a senior front-end developer with 5+ years of experience in Canvas API and pixel art tools. Design a step-by-step guide for building a [SIZE]x[SIZE] pixel art editor using HTML5 Canvas and JavaScript. Include features like [COLOR_PICKER_TYPE] (e.g., hex/rgb), [BRUSH_SIZE] adjustment, layer support, and export to PNG. Explain how to implement efficient rendering for smooth performance, handle mouse/touch events for drawing, and add undo/redo functionality using a [DATA_STRUCTURE] (e.g., stack or array). Provide clean, modular code examples with comments and optimize for mobile responsiveness. Mention any key Canvas API methods like getImageData() and putImageData().