feat(ui): spinners for ConPTY session start and task refine

Both actions previously gave no feedback: opening a ConPTY session only created
the tile after the launch-spec roundtrip (which may build a worktree), and the
refine button just disappeared while the run was in flight.

Add a shared Ellipse.spinner style, and let ConPtyPaneViewModel resolve its own
launch spec so the tile shows up immediately with a starting overlay. A failed
launch now keeps the tile with its inline error banner instead of never
appearing — Start() is separated from the ctor so the host can subscribe to
ErrorReported before the launch begins.

src/ClaudeDo.Ui/ViewModels/InteractiveTerminalViewModel.cs

@@ -22,6 +22,15 @@ public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDispo
     [ObservableProperty] private int? _exitCode;
     [ObservableProperty] private string? _startError;
 
+    /// <summary>True from construction until the child process is actually launched (or the launch
+    /// failed) — covers both the caller's launch-spec roundtrip and the ConPTY spawn, so the host
+    /// can show a spinner instead of an empty black pane.</summary>
+    public bool IsStarting => !IsRunning && !HasExited && StartError is null;
+
+    partial void OnIsRunningChanged(bool value)   => OnPropertyChanged(nameof(IsStarting));
+    partial void OnHasExitedChanged(bool value)   => OnPropertyChanged(nameof(IsStarting));
+    partial void OnStartErrorChanged(string? value) => OnPropertyChanged(nameof(IsStarting));
+
     public InteractiveTerminalViewModel()
     {
         _session.ProcessExited += OnSessionProcessExited;
@@ -74,6 +83,15 @@ public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDispo
         ExitCode = exitCode;
     }
 
+    /// <summary>Reports a failure that happened before <see cref="Start"/> could be called (e.g. the
+    /// launch-spec roundtrip threw), so it surfaces through the same banner as a spawn failure.</summary>
+    public void ReportStartFailure(string message)
+    {
+        IsRunning = false;
+        HasExited = true;
+        StartError = message;
+    }
+
     public void Kill() => _session.Kill();
 
     public void Dispose()