fix(ui): gate Mission Control submit-for-review, add retry, fix event leak

Submit for Review is now disabled while a ConPTY pane is starting, has
failed to launch, or has already exited, and MissionControlViewModel
guards against a rapid double-click racing two SubmitTaskForReviewAsync
calls. A failed launch no longer permanently occupies its TaskId dedupe
slot -- a Retry button re-fetches the launch spec and restarts the pane
in place. CloseConPtySession/Dispose now also unsubscribe
SubmitForReviewRequested, matching the other pane event handlers. Also
fixes the pre-existing nullable-dereference warning in
IslandsShellViewModel.SyncInteractiveSessionChips.

src/ClaudeDo.Ui/ViewModels/MissionControlViewModel.cs

@@ -302,16 +302,22 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
 
     // Submit a task's hand-driven ConPTY work for review, then close the pane (the interactive
     // session is finished). The worker commits the worktree and moves the task to WaitingForReview.
+    // Guarded by the pane's IsSubmitPending flag — a rapid double-click would otherwise race two
+    // SubmitTaskForReviewAsync calls, with the loser flashing a spurious footer error.
     private async void OnPaneSubmitForReview(string taskId)
     {
+        if (ConPtySessions.FirstOrDefault(s => s.TaskId == taskId) is not { } pane || pane.IsSubmitPending)
+            return;
+
+        pane.IsSubmitPending = true;
         try
         {
             await _worker.SubmitTaskForReviewAsync(taskId);
-            if (ConPtySessions.FirstOrDefault(s => s.TaskId == taskId) is { } pane)
-                CloseConPtySession(pane);
+            CloseConPtySession(pane);
         }
         catch (Exception ex)
         {
+            pane.IsSubmitPending = false;
             ErrorReported?.Invoke(Loc.T("missionControl.submitForReviewFailed", ex.Message));
         }
     }
@@ -321,6 +327,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         if (!ConPtySessions.Contains(pane)) return;
         pane.ErrorReported -= OnConPtyPaneError;
         pane.CloseRequested -= CloseConPtySession;
+        pane.SubmitForReviewRequested -= OnPaneSubmitForReview;
         ConPtySessions.Remove(pane);
         pane.Dispose();
     }
@@ -370,6 +377,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         {
             c.ErrorReported -= OnConPtyPaneError;
             c.CloseRequested -= CloseConPtySession;
+            c.SubmitForReviewRequested -= OnPaneSubmitForReview;
             c.Dispose();
         }
         ConPtySessions.Clear();