Give the window its v5b chrome and each session surface its own shell

src/DodoSSH.Client.Shell/ViewModels/TerminalTabViewModel.cs

@@ -88,6 +88,20 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
     /// <summary>The account and endpoint, for the pane header and the status bar.</summary>
     internal string Address { get; }
 
+    /// <summary>
+    /// When the session behind this tab opened, for the status bar's elapsed timer — or null while there is
+    /// none.
+    /// </summary>
+    /// <remarks>
+    /// Set by <c>MainWindowViewModel</c> from its own clock at the same moment it calls <see cref="Opened"/>
+    /// or constructs a tab that already carries a session id, rather than read here from
+    /// <see cref="TimeProvider.System"/> directly — a view model with its own notion of "now" is a second
+    /// clock the shell's tests would have no way to control. Left on a tab whose session has since ended:
+    /// the pane still holds the scrollback, and "how long that shell ran" stays a true fact about it after it
+    /// stops being live.
+    /// </remarks>
+    internal DateTimeOffset? StartedAt { get; set; }
+
     /// <inheritdoc cref="TerminalTabState" />
     [ObservableProperty]
     private TerminalTabState state;