feat(HAI-053): add responsive mobile horizontal scroll for board

- Add responsive horizontal scroll CSS styles for mobile viewports
- Ensure board columns scroll horizontally on smaller screens
- Add Board component tests verifying responsive scroll behavior

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

@@ -0,0 +1,48 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { Board } from "../Board";
+import { COLUMNS } from "@hai/core";
+
+// Mock child components so we only test Board's own rendering
+vi.mock("../Column", () => ({
+  Column: ({ column }: { column: string }) => (
+    <div data-testid={`column-${column}`} />
+  ),
+}));
+
+const noop = () => {};
+const noopAsync = () => Promise.resolve({} as any);
+
+function renderBoard() {
+  return render(
+    <Board
+      tasks={[]}
+      maxConcurrent={2}
+      onMoveTask={noopAsync}
+      onOpenDetail={noop}
+      addToast={noop}
+      isCreating={false}
+      onCancelCreate={noop}
+      onCreateTask={noopAsync}
+      onNewTask={noop}
+      autoMerge={false}
+      onToggleAutoMerge={noop}
+    />,
+  );
+}
+
+describe("Board", () => {
+  it("renders a <main> element with class 'board'", () => {
+    renderBoard();
+    const main = screen.getByRole("main");
+    expect(main).toBeDefined();
+    expect(main.className).toContain("board");
+  });
+
+  it("renders all 5 columns", () => {
+    renderBoard();
+    for (const col of COLUMNS) {
+      expect(screen.getByTestId(`column-${col}`)).toBeDefined();
+    }
+  });
+});