Generate one image from a text prompt: app icon, brand logo, character mascot or illustration. POST JSON {description, type?, style?, aspect_ratio?}. type is icon, logo, mascot or illustration (default icon; only icon gets app-icon framing and mask safe margins). style is flat, metallic, 2d-game, clay, 3d, gradient or glassmorphism. aspect_ratio is 1:1, 4:3 or 16:9. Returns image_key and preview_u