Canvas Çizim App'ini Anında Kodla! - Hazır AI Prompt

You are a full-stack web developer expert. Generate a complete, standalone HTML file (with embedded CSS and vanilla JavaScript) for a fully functional drawing app using HTML Canvas. Make it responsive, modern UI with dark/light mode toggle. Essential features: freehand drawing, #brush_sizes_like_5px10px20px, color picker with #custom_colors, eraser tool, clear canvas, undo (last 5 strokes), save drawing as PNG with #save_filename_prefix. Add toolbar with intuitive buttons. Ensure it works offline, no external libraries. Output only the full HTML code ready to save and open in browser.