rustfmt, clippy, gh actions, justfile, Default app (#4)

* `Default` app
* add zed settings
* add justfile
* flake: update devShell, ignore tests, add `just`
* update README
* gh actions: lint, format, tests. build

src/main.rs

@@ -11,7 +11,7 @@ use color_eyre::{eyre::Context, Result};
 fn main() -> Result<()> {
     color_eyre::install()?;
     let terminal = ratatui::init();
-    let app_result = App::new().run(terminal).context("app loop failed");
+    let app_result = App::default().run(terminal).context("app loop failed");
     ratatui::restore();
     app_result
 }