feat(KB-175): move file browser to header with workspace support

- Add workspace file APIs and hooks for session-scoped file browsing
- Create WorkspaceSelector component for switching between workspaces
- Update FileBrowserModal with workspace mode and improved UX
- Add files button to header with workspace-aware file browsing
- Add session files indicator to task cards for quick file access
- Include comprehensive tests for all new components and hooks

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

@@ -0,0 +1,74 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, act, waitFor } from "@testing-library/react";
+import { useWorkspaceFileBrowser } from "../useWorkspaceFileBrowser";
+import * as api from "../../api";
+import type { FileListResponse } from "../../api";
+
+vi.mock("../../api", () => ({
+  fetchWorkspaceFileList: vi.fn(),
+}));
+
+const mockFetchWorkspaceFileList = vi.mocked(api.fetchWorkspaceFileList);
+
+describe("useWorkspaceFileBrowser", () => {
+  beforeEach(() => {
+    mockFetchWorkspaceFileList.mockReset();
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("fetches workspace files when enabled", async () => {
+    const mockResponse: FileListResponse = {
+      path: ".",
+      entries: [{ name: "src", type: "directory", mtime: "2024-01-01T00:00:00Z" }],
+    };
+    mockFetchWorkspaceFileList.mockResolvedValueOnce(mockResponse);
+
+    const { result } = renderHook(() => useWorkspaceFileBrowser("KB-123", true));
+
+    expect(result.current.loading).toBe(true);
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.entries).toEqual(mockResponse.entries);
+    expect(mockFetchWorkspaceFileList).toHaveBeenCalledWith("KB-123", undefined);
+  });
+
+  it("resets path when workspace changes", async () => {
+    mockFetchWorkspaceFileList
+      .mockResolvedValueOnce({ path: ".", entries: [{ name: "src", type: "directory", mtime: "2024-01-01T00:00:00Z" }] })
+      .mockResolvedValueOnce({ path: "src", entries: [{ name: "index.ts", type: "file", size: 1, mtime: "2024-01-01T00:00:00Z" }] })
+      .mockResolvedValueOnce({ path: ".", entries: [{ name: "README.md", type: "file", size: 5, mtime: "2024-01-01T00:00:00Z" }] });
+
+    const { result, rerender } = renderHook(
+      ({ workspace }) => useWorkspaceFileBrowser(workspace, true),
+      { initialProps: { workspace: "KB-123" } },
+    );
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    act(() => {
+      result.current.setPath("src");
+    });
+
+    await waitFor(() => expect(result.current.currentPath).toBe("src"));
+    await waitFor(() => expect(mockFetchWorkspaceFileList).toHaveBeenLastCalledWith("KB-123", "src"));
+
+    rerender({ workspace: "project" });
+
+    await waitFor(() => expect(result.current.currentPath).toBe("."));
+    await waitFor(() => expect(mockFetchWorkspaceFileList).toHaveBeenLastCalledWith("project", undefined));
+  });
+
+  it("handles fetch errors", async () => {
+    mockFetchWorkspaceFileList.mockRejectedValueOnce(new Error("Failed to load files"));
+
+    const { result } = renderHook(() => useWorkspaceFileBrowser("KB-404", true));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.error).toBe("Failed to load files");
+    expect(result.current.entries).toEqual([]);
+  });
+});