feat(HAI-055): add failed task status handling with retry support

- Set task status to 'failed' on execution failure in engine executor
- Add failed indicator styling on TaskCard component
- Add POST /tasks/:id/retry API endpoint and client function
- Add retry button in TaskDetailModal for failed tasks
- Add tests for failed indicator, retry endpoint, and modal behavior

packages/dashboard/app/components/__tests__/TaskDetailModal.test.tsx

@@ -22,6 +22,7 @@ const noop = vi.fn();
 const noopMove = vi.fn(async () => ({}) as Task);
 const noopDelete = vi.fn(async () => ({}) as Task);
 const noopMerge = vi.fn(async () => ({ merged: false }) as MergeResult);
+const noopRetry = vi.fn(async () => ({}) as Task);
 
 describe("TaskDetailModal", () => {
   it("renders markdown-body without detail-prompt class when prompt exists", () => {
@@ -91,6 +92,53 @@ describe("TaskDetailModal", () => {
     expect(screen.queryByText("PROMPT.md")).toBeNull();
   });
 
+  it("renders Retry button when task status is 'failed'", () => {
+    render(
+      <TaskDetailModal
+        task={makeTask({ status: "failed" })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        onRetryTask={noopRetry}
+        addToast={noop}
+      />,
+    );
+
+    expect(screen.getByText("Retry")).toBeTruthy();
+  });
+
+  it("does NOT render Retry button when task status is not 'failed'", () => {
+    render(
+      <TaskDetailModal
+        task={makeTask({ status: "executing" })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        onRetryTask={noopRetry}
+        addToast={noop}
+      />,
+    );
+
+    expect(screen.queryByText("Retry")).toBeNull();
+  });
+
+  it("does NOT render Retry button when onRetryTask is not provided", () => {
+    render(
+      <TaskDetailModal
+        task={makeTask({ status: "failed" })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    expect(screen.queryByText("Retry")).toBeNull();
+  });
+
   it("shows description exactly once for a task without title", () => {
     const { container } = render(
       <TaskDetailModal