KDE/krita ? reverse-engineered prompt

Build me a cross platform digital painting app like Krita for artists who want to sketch, paint, and finish artwork in one place. I want a clean desktop app with a canvas, basic brush tools, color picking, layers, undo and redo, and the ability to open and save common image files. Make the interface feel friendly for illustrators and concept artists, with a professional but simple workflow that does not get in the way of drawing.

Please set it up so it runs as a real app from source, and if you need current build or framework details, look them up online. Focus on the main painting experience first, with a few useful starter brushes and a simple example document, and make sure the app launches cleanly on desktop platforms.