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`

src/widgets/countdown.rs

@@ -0,0 +1,23 @@
+use ratatui::{
+    buffer::Buffer,
+    layout::Rect,
+    widgets::{Paragraph, Widget},
+};
+
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+pub struct Countdown {
+    headline: String,
+}
+
+impl Countdown {
+    pub const fn new(headline: String) -> Self {
+        Self { headline }
+    }
+}
+
+impl Widget for Countdown {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let h = Paragraph::new(self.headline).centered();
+        h.render(area, buf);
+    }
+}