Exit after --recode-samples

Otherwise it may leave the app in a weird state where
app.image != app.canvas.image (I presume), and you can't draw, at least
not visibly, and it can prompt you to save changes to the document
that isn't visibly open, except in the titlebar (Header).

src/textual_paint/paint.py

@@ -3440,6 +3440,7 @@ if args.recode_samples:
                 app.file_path = os.path.abspath(os.path.join("samples", filename))
             await app.save()
             print(f"Saved {filename}")
+        app.exit()
     # asyncio.run(recode_samples())
     # have to wait for the app to be initialized
     def callback() -> None: