Improve dx fmt and add to treefmt

[srid]

See https://dioxuslabs.com/blog/release-030#autoformatting

The dx fmt command in the dev shell of #79 can be used to format the render! HTML DSL functions, but it is pretty limited.

The PR's justfile defines just fmt like this:

# Auto-format the source tree
fmt:
    treefmt
    find src/app/ -name \*.rs | grep -v state.rs | xargs -n 1 dx fmt -f

Our goal here is to remove that line, so that just fmt runs nothing but treefmt.

To do this, two things need to happen:

1. Open a PR on https://github.com/numtide/treefmt-nix adding the dioxus formatter. Use the leptos PR for reference: Add leptosfmt numtide/treefmt-nix#108
   • Note: Dioxus is available in nixpkgs, as pkgs.dioxus-cli
2. Create corresponding PRs on the dioxus repo to improve integration with treefmt, just like we did with leptosfmt for the aforementioned PR