Desktop: stop the status bar's session timer when the shell ends

tests/DodoSSH.Client.App.Tests/ShellFlowTests.cs

@@ -2075,6 +2075,15 @@ public sealed partial class ShellFlowTests : IAsyncLifetime
         // Still worth offering, because the tab and the machine are both still there — what it is not is an
         // alarm.
         shell.CanReconnectSelectedTab.ShouldBeTrue();
+
+        // And the timer stopped with it: how long that shell ran, rather than how long ago it started. The
+        // shell's clock here is the system's, so the second has to actually pass for the two to differ.
+        await Task.Delay(TimeSpan.FromSeconds(1.2), Token);
+
+        var ran = tab.EndedAt.ShouldNotBeNull() - tab.StartedAt.ShouldNotBeNull();
+
+        shell.SessionElapsedText.ShouldBe(string.Create(
+            CultureInfo.InvariantCulture, $"session {(int)ran.TotalHours:00}:{ran.Minutes:00}:{ran.Seconds:00}"));
     }
 
     /// <remarks>