Event handling (#5)

- Refactor `event` handling (heavily inspired by [crates-tui](https://github.com/ratatui/crates-tui/) via [Tui with Terminal and EventHandler](https://ratatui.rs/recipes/apps/terminal-and-event-handler/))
- Refactor widget structure
- Disable `nixos-unstable` temporarily
- Add `.rustfmt.toml`

justfile

@@ -1,8 +1,9 @@
 # The `--fmt` command is currently unstable.
 
-set unstable := true
+# set unstable := true
 
-default: run
+default:
+    @just --list
 
 alias b := build
 alias f := format
@@ -20,7 +21,7 @@ test:
 
 # format files
 format:
-    just --fmt
+    # just --fmt
     cargo fmt --check
 
 # lint