feat(FN-1070): restore desktop Electron scaffolding

- Reintroduce the @fusion/desktop workspace package with Electron main, preload, and renderer entrypoints
- Add desktop build/test tooling (TypeScript, Vitest, electron-builder) and update workspace build dependency settings for Electron
- Wire secure IPC bridge APIs for app version/quit flows with exported preload typings
- Add README usage docs and unit tests covering main-process window/tray behavior and preload API wiring

packages/desktop/src/__tests__/preload.test.ts

@@ -0,0 +1,94 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const mocks = vi.hoisted(() => {
+  const contextBridge = {
+    exposeInMainWorld: vi.fn(),
+  };
+
+  const ipcRenderer = {
+    invoke: vi.fn(),
+    send: vi.fn(),
+    on: vi.fn(),
+    removeListener: vi.fn(),
+  };
+
+  return { contextBridge, ipcRenderer };
+});
+
+vi.mock("electron", () => ({
+  contextBridge: mocks.contextBridge,
+  ipcRenderer: mocks.ipcRenderer,
+}));
+
+async function importPreloadModule() {
+  await import("../preload.ts");
+}
+
+function getExposedApi() {
+  const call = mocks.contextBridge.exposeInMainWorld.mock.calls[0] as
+    | [string, {
+      getAppVersion: () => Promise<string>;
+      quit: () => void;
+      onDashboardReady: (callback: () => void) => () => void;
+    }]
+    | undefined;
+
+  return call?.[1];
+}
+
+describe("preload", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.resetModules();
+  });
+
+  it("contextBridge.exposeInMainWorld called with fusionDesktop", async () => {
+    await importPreloadModule();
+
+    expect(mocks.contextBridge.exposeInMainWorld).toHaveBeenCalledWith(
+      "fusionDesktop",
+      expect.any(Object),
+    );
+  });
+
+  it("getAppVersion calls ipcRenderer.invoke", async () => {
+    mocks.ipcRenderer.invoke.mockResolvedValue("0.1.0");
+    await importPreloadModule();
+
+    const api = getExposedApi();
+    const version = await api?.getAppVersion();
+
+    expect(version).toBe("0.1.0");
+    expect(mocks.ipcRenderer.invoke).toHaveBeenCalledWith("app:get-version");
+  });
+
+  it("quit calls ipcRenderer.send", async () => {
+    await importPreloadModule();
+
+    const api = getExposedApi();
+    api?.quit();
+
+    expect(mocks.ipcRenderer.send).toHaveBeenCalledWith("app:quit");
+  });
+
+  it("onDashboardReady returns unsubscribe function", async () => {
+    await importPreloadModule();
+
+    const api = getExposedApi();
+    const callback = vi.fn();
+    const unsubscribe = api?.onDashboardReady(callback);
+
+    expect(mocks.ipcRenderer.on).toHaveBeenCalledWith(
+      "dashboard:ready",
+      expect.any(Function),
+    );
+    expect(typeof unsubscribe).toBe("function");
+
+    unsubscribe?.();
+
+    expect(mocks.ipcRenderer.removeListener).toHaveBeenCalledWith(
+      "dashboard:ready",
+      expect.any(Function),
+    );
+  });
+});