refactor: annotate State::allocate_image with must_use

src/main.rs

@@ -158,6 +158,7 @@ impl State {
         self.error = utils::save_image(self.image.as_ref());
     }
 
+    #[must_use]
     fn allocate_image(&self) -> Task<Message> {
         let Some(image) = self.image.as_ref() else {
             eprintln!("no image to allocate");