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)

flake.nix

@@ -39,7 +39,6 @@
             pkg-config
           ];
           buildInputs = with pkgs; [
-            libjxl
             dav1d
           ];