feat: switch to pure-rust jxl decoder

this makes the logic drastically simpler;
as it comes with image crate integration.

however, images can no longer be saved as JXL with this version
(as jxl-oxide is decoder-only)

src/main.rs

@@ -175,6 +175,8 @@ impl State {
 }
 
 fn main() -> Result<(), iced::Error> {
+    jxl_oxide::integration::register_image_decoding_hook();
+
     iced::application(State::new, State::update, State::view)
         .subscription(State::subscription)
         .title(State::title)