Build a paint / drawing web app as a single page in this directory: index.html, style.css, paint.js (vanilla JS, no deps, HTML canvas). Freehand brush with adjustable size and color, a color palette plus a custom color picker, an eraser, a clear-canvas button, undo/redo, and a "save as PNG" download. Show the current brush preview. Polished dark theme, responsive canvas. Make it feel good to draw on.