fix(HAI-018): complete Step 2 — add client-side retry for fetchTaskDetail

- Added withRetry wrapper in api.ts (1 retry, 200ms delay)
- fetchTaskDetail now retries transient 500s transparently
- TaskCard shows 'Failed to load task details' instead of raw error
- Added api.test.ts with success, retry-success, and exhaustion tests

packages/dashboard/app/api.test.ts

@@ -0,0 +1,65 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { fetchTaskDetail } from "./api";
+import type { TaskDetail } from "@hai/core";
+
+const FAKE_DETAIL: TaskDetail = {
+  id: "HAI-001",
+  description: "Test",
+  column: "in-progress",
+  dependencies: [],
+  steps: [],
+  currentStep: 0,
+  log: [],
+  createdAt: "2026-01-01T00:00:00.000Z",
+  updatedAt: "2026-01-01T00:00:00.000Z",
+  prompt: "# HAI-001",
+};
+
+function mockFetchResponse(ok: boolean, body: unknown, status = ok ? 200 : 500) {
+  return Promise.resolve({
+    ok,
+    status,
+    json: () => Promise.resolve(body),
+  } as Response);
+}
+
+describe("fetchTaskDetail", () => {
+  const originalFetch = globalThis.fetch;
+
+  beforeEach(() => {
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+  });
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+    vi.useRealTimers();
+  });
+
+  it("returns data on first success", async () => {
+    globalThis.fetch = vi.fn().mockReturnValue(mockFetchResponse(true, FAKE_DETAIL));
+
+    const result = await fetchTaskDetail("HAI-001");
+
+    expect(result.id).toBe("HAI-001");
+    expect(globalThis.fetch).toHaveBeenCalledTimes(1);
+  });
+
+  it("retries once on failure then succeeds", async () => {
+    globalThis.fetch = vi.fn()
+      .mockReturnValueOnce(mockFetchResponse(false, { error: "Transient error" }))
+      .mockReturnValueOnce(mockFetchResponse(true, FAKE_DETAIL));
+
+    const result = await fetchTaskDetail("HAI-001");
+
+    expect(result.id).toBe("HAI-001");
+    expect(globalThis.fetch).toHaveBeenCalledTimes(2);
+  });
+
+  it("throws after retry exhaustion", async () => {
+    globalThis.fetch = vi.fn()
+      .mockReturnValue(mockFetchResponse(false, { error: "Server error" }));
+
+    await expect(fetchTaskDetail("HAI-001")).rejects.toThrow("Server error");
+    expect(globalThis.fetch).toHaveBeenCalledTimes(2); // initial + 1 retry
+  });
+});