Add last_failure to worker state

The `WorkerSnapshot` now includes `last_failure`, which tracks the time
of the last recorded error. This allows the UI to display a more
accurate status, especially when a failure occurs after a recent
success.

The `pick_footer_status` function has been updated to prioritize
`last_failure`. If a failure occurred more recently than the last
success, the footer will show `Offline`, regardless of the success age.
This addresses scenarios where the server might have temporarily failed,
and the UI should reflect that immediately.

The `run_loop` function now updates `last_failure` upon transient or
terminal upload errors and poll failures. This ensures that the new
state information is correctly propagated.

client-desktop/src/app.rs

@@ -524,7 +524,8 @@ impl DoctateApp {
             return;
         };
         let snap = worker_rx.borrow().clone();
-        let last_age = snap.last_success.map(|i| i.elapsed());
+        let last_success_age = snap.last_success.map(|i| i.elapsed());
+        let last_failure_age = snap.last_failure.map(|i| i.elapsed());
         let threshold = self
             .config
             .as_ref()
@@ -535,7 +536,8 @@ impl DoctateApp {
             self.finalizing.is_some(),
             snap.state,
             snap.queue_len,
-            last_age,
+            last_success_age,
+            last_failure_age,
             threshold,
         );