feat(tui): use moving heart emoji as liveness indicator

The heart moves between two positions every second as long as the ui is
being updated.

crates/cli/src/tui/actor/model.rs

@@ -40,6 +40,15 @@ impl State {
             *tick += 1;
         }
     }
+
+    fn beating_heart(&self) -> String {
+        if self.last_update.duration_since(self.started).as_secs() % 2 == 0 {
+            "💚 "
+        } else {
+            " 💚"
+        }
+        .to_string()
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -326,11 +335,8 @@ impl StatefulWidget for &State {
             .title(
                 Title::from(Line::from(vec![
                     " [q]uit ".into(),
-                    format!(
-                        "{}s ",
-                        self.last_update.duration_since(self.started).as_secs()
-                    )
-                    .into(),
+                    self.beating_heart().into(),
+                    " ".into(),
                 ]))
                 .alignment(Alignment::Center)
                 .position(ratatui::widgets::block::Position::Bottom),