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/footer.rs

@@ -0,0 +1,71 @@
+use std::collections::BTreeMap;
+
+use crate::app::Content;
+use ratatui::{
+    buffer::Buffer,
+    layout::{Constraint, Layout, Rect},
+    style::{Modifier, Style},
+    symbols,
+    text::{Line, Span},
+    widgets::{Block, Borders, Widget},
+};
+
+#[derive(Debug, Clone)]
+pub struct Footer {
+    show_menu: bool,
+    selected_content: Content,
+    content_labels: BTreeMap<Content, String>,
+}
+
+impl Footer {
+    pub fn new(show_menu: bool, selected_content: Content) -> Self {
+        Self {
+            show_menu,
+            selected_content,
+            content_labels: BTreeMap::from([
+                (Content::Countdown, "[c]ountdown".into()),
+                (Content::Timer, "[t]imer".into()),
+                (Content::Pomodoro, "[p]omodoro".into()),
+            ]),
+        }
+    }
+}
+
+impl Widget for Footer {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let [border_area, menu_area] =
+            Layout::vertical([Constraint::Length(1), Constraint::Fill(0)]).areas(area);
+        Block::new()
+            .borders(Borders::TOP)
+            .title(format! {"[m]enu {:} ", if self.show_menu {"↓"} else {"↑"}})
+            .border_set(symbols::border::DOUBLE)
+            .render(border_area, buf);
+        // show menu
+        if self.show_menu {
+            let [title_area, labels_area] =
+                Layout::horizontal([Constraint::Length(12), Constraint::Fill(0)]).areas(menu_area);
+
+            Span::styled("screens", Style::default().add_modifier(Modifier::BOLD))
+                .render(title_area, buf);
+            let spans: Vec<Span> = self
+                .content_labels
+                .iter()
+                .enumerate()
+                .map(|(index, (content, label))| {
+                    let mut style = Style::default();
+                    // Add space for all except last
+                    let label = if index < self.content_labels.len() - 1 {
+                        format!("{}  ", label)
+                    } else {
+                        label.into()
+                    };
+                    if *content == self.selected_content {
+                        style = style.add_modifier(Modifier::BOLD);
+                    }
+                    Span::styled(label, style)
+                })
+                .collect();
+            Line::from(spans).render(labels_area, buf);
+        }
+    }
+}