Fix status bar timer: use activity start time, tick every 1s

The status bar timer was showing turn/call elapsed times (0s, 0/60s)
instead of the activity's actual elapsed time. Use activity_started
from the ActivityEntry directly.

Add a 1s tick to the UI select loop when an activity is active so
the timer updates live.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/mod.rs

@@ -358,7 +358,11 @@ async fn run(
     let mut startup_done = false;
     let mut dirty = true; // render on first loop
 
+    let mut activity_tick = tokio::time::interval(std::time::Duration::from_secs(1));
+    activity_tick.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
+
     loop {
+        let has_activity = !app.activity.is_empty();
         tokio::select! {
             biased;
 
@@ -380,6 +384,10 @@ async fn run(
             Some(channels) = channel_rx.recv() => {
                 app.set_channel_status(channels);
             }
+
+            _ = activity_tick.tick(), if has_activity => {
+                dirty = true;
+            }
         }
 
         // State sync on every wake