feat(FN-950): add useTaskDiffStats hook for consistent done file count in TaskCard

- Create useTaskDiffStats hook to extract and compute diff stats from task data
- Use hook in TaskCard component for consistent done file count display
- Add diffStats priority tests ensuring diff stats take precedence over steps
- Add useTaskDiffStats mock and comprehensive unit tests for the hook

packages/dashboard/app/hooks/__tests__/useTaskDiffStats.test.ts

@@ -0,0 +1,162 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, waitFor } from "@testing-library/react";
+import { useTaskDiffStats } from "../useTaskDiffStats";
+import * as api from "../../api";
+
+vi.mock("../../api", () => ({
+  fetchTaskDiff: vi.fn(),
+}));
+
+const mockFetchTaskDiff = vi.mocked(api.fetchTaskDiff);
+
+describe("useTaskDiffStats", () => {
+  beforeEach(() => {
+    mockFetchTaskDiff.mockReset();
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("fetches diff stats for done tasks with a commit SHA", async () => {
+    mockFetchTaskDiff.mockResolvedValueOnce({
+      files: [
+        { path: "src/a.ts", status: "modified", additions: 10, deletions: 2, patch: "" },
+        { path: "src/b.ts", status: "added", additions: 5, deletions: 0, patch: "" },
+      ],
+      stats: { filesChanged: 2, additions: 15, deletions: 2 },
+    });
+
+    const { result } = renderHook(() =>
+      useTaskDiffStats("FN-123", "done", "abc1234", undefined),
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.stats).toEqual({ filesChanged: 2, additions: 15, deletions: 2 });
+    expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-123", undefined, undefined);
+  });
+
+  it("passes projectId to fetchTaskDiff", async () => {
+    mockFetchTaskDiff.mockResolvedValueOnce({
+      files: [],
+      stats: { filesChanged: 0, additions: 0, deletions: 0 },
+    });
+
+    const { result } = renderHook(() =>
+      useTaskDiffStats("FN-123", "done", "abc1234", "proj-1"),
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-123", undefined, "proj-1");
+  });
+
+  it("does not fetch for non-done columns", async () => {
+    const { result: inProgress } = renderHook(() =>
+      useTaskDiffStats("FN-123", "in-progress", "abc1234", undefined),
+    );
+    const { result: todo } = renderHook(() =>
+      useTaskDiffStats("FN-123", "todo", "abc1234", undefined),
+    );
+    const { result: inReview } = renderHook(() =>
+      useTaskDiffStats("FN-123", "in-review", "abc1234", undefined),
+    );
+
+    await waitFor(() => expect(inProgress.current.loading).toBe(false));
+    await waitFor(() => expect(todo.current.loading).toBe(false));
+    await waitFor(() => expect(inReview.current.loading).toBe(false));
+
+    expect(inProgress.current.stats).toBeNull();
+    expect(todo.current.stats).toBeNull();
+    expect(inReview.current.stats).toBeNull();
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+  });
+
+  it("does not fetch for done tasks without a commit SHA", async () => {
+    const { result } = renderHook(() =>
+      useTaskDiffStats("FN-123", "done", undefined, undefined),
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.stats).toBeNull();
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+  });
+
+  it("does not fetch for empty task ID", async () => {
+    const { result } = renderHook(() =>
+      useTaskDiffStats("", "done", "abc1234", undefined),
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.stats).toBeNull();
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+  });
+
+  it("returns null stats on fetch failure", async () => {
+    mockFetchTaskDiff.mockRejectedValueOnce(new Error("Network error"));
+
+    const { result } = renderHook(() =>
+      useTaskDiffStats("FN-123", "done", "abc1234", undefined),
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.stats).toBeNull();
+  });
+
+  it("cancels in-flight request on dependency change", async () => {
+    let resolveFirst: (value: unknown) => void;
+    const firstPromise = new Promise((resolve) => {
+      resolveFirst = resolve;
+    });
+    mockFetchTaskDiff.mockReturnValueOnce(firstPromise as any);
+    mockFetchTaskDiff.mockResolvedValueOnce({
+      files: [],
+      stats: { filesChanged: 3, additions: 5, deletions: 1 },
+    });
+
+    const { result, rerender } = renderHook(
+      ({ taskId }) => useTaskDiffStats(taskId, "done", "abc1234", undefined),
+      { initialProps: { taskId: "FN-100" } },
+    );
+
+    // Rerender with a different taskId before the first fetch resolves
+    rerender({ taskId: "FN-200" });
+
+    // Resolve the first (now cancelled) request
+    resolveFirst!({
+      files: [],
+      stats: { filesChanged: 99, additions: 99, deletions: 99 },
+    });
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    // The cancelled response should not have been stored
+    expect(result.current.stats).toEqual({ filesChanged: 3, additions: 5, deletions: 1 });
+    expect(mockFetchTaskDiff).toHaveBeenCalledTimes(2);
+  });
+
+  it("resets stats when column changes from done to non-done", async () => {
+    mockFetchTaskDiff.mockResolvedValueOnce({
+      files: [],
+      stats: { filesChanged: 5, additions: 10, deletions: 3 },
+    });
+
+    const { result, rerender } = renderHook(
+      ({ column }) => useTaskDiffStats("FN-123", column, "abc1234", undefined),
+      { initialProps: { column: "done" as string } },
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+    expect(result.current.stats).toEqual({ filesChanged: 5, additions: 10, deletions: 3 });
+
+    // Switch to a non-done column
+    rerender({ column: "in-progress" });
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+    expect(result.current.stats).toBeNull();
+  });
+});