fix(review): propagate HubException from ApproveReviewAsync so blocked merges surface errors

TryInvokeAsync swallowed all exceptions including HubException, so a
blocked merge (uncommitted changes in target, mid-merge state, inactive
worktree) returned null silently — the task stayed WaitingForReview with
no feedback shown.  Switch to a direct _hub.InvokeAsync so both VM
catch blocks (TasksIsland ErrorReported, DetailsIsland ShowErrorAsync)
actually fire.

Add regression tests for both call sites verifying that a throwing
worker causes the error to be reported.

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -450,10 +450,10 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         await _hub.InvokeAsync("SetTaskStatus", taskId, status.ToString());
     }
 
-    public Task<MergeResultDto?> ApproveReviewAsync(string taskId, string targetBranch)
+    public async Task<MergeResultDto?> ApproveReviewAsync(string taskId, string targetBranch)
     {
         LastApproveTarget = targetBranch;
-        return TryInvokeAsync<MergeResultDto>("ApproveReview", taskId, targetBranch);
+        return await _hub.InvokeAsync<MergeResultDto>("ApproveReview", taskId, targetBranch);
     }
 
     public Task<MergePreviewDto?> PreviewMergeAsync(string taskId, string targetBranch)