fix: Flatten RGBA images before printing

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

main.py

@@ -31,6 +31,11 @@ def print_picture(filename, p):
 
         img = Image.open(io.BytesIO(response.content))
 
+        if img.mode == 'RGBA':
+            bg = Image.new('RGB', img.size, (255, 255, 255))
+            bg.paste(img, mask=img.split()[3])
+            img = bg
+
         # Convert with dithering
         img = img.convert('1', dither=Image.FLOYDSTEINBERG)