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/README.md

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+# @fusion/desktop
+
+Electron desktop shell for Fusion.
+
+This package provides a native Electron wrapper around the existing Fusion dashboard web UI. The desktop shell currently connects to a running dashboard server and displays it inside a desktop window with tray integration.
+
+## Prerequisites
+
+Start the Fusion dashboard server first:
+
+```bash
+fn dashboard
+```
+
+Then, in another terminal, start the desktop app:
+
+```bash
+pnpm --filter @fusion/desktop dev
+```
+
+## Scripts
+
+- `pnpm --filter @fusion/desktop dev` — run the Electron main process in development
+- `pnpm --filter @fusion/desktop build` — compile TypeScript sources
+- `pnpm --filter @fusion/desktop test` — run Vitest suite
+- `pnpm --filter @fusion/desktop typecheck` — run TypeScript checks without emitting files
+- `pnpm --filter @fusion/desktop pack` — build distributable package via electron-builder
+- `pnpm --filter @fusion/desktop dist` — build distribution artifacts without publishing
+
+## Environment
+
+- `FUSION_DASHBOARD_URL` — override the default dashboard URL used by the desktop shell (`http://localhost:4040`)

packages/desktop/package.json

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+{
+  "name": "@fusion/desktop",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "src/main.ts",
+  "engines": {
+    "node": ">=22.5.0"
+  },
+  "scripts": {
+    "dev": "tsx src/main.ts",
+    "build": "tsc",
+    "test": "vitest run",
+    "typecheck": "tsc --noEmit",
+    "pack": "electron-builder",
+    "dist": "electron-builder --publish never"
+  },
+  "build": {
+    "appId": "com.fusion.desktop",
+    "productName": "Fusion",
+    "directories": {
+      "output": "dist-electron"
+    },
+    "mac": {
+      "category": "public.app-category.developer-tools",
+      "target": [
+        "dmg"
+      ]
+    },
+    "win": {
+      "target": [
+        "nsis"
+      ]
+    },
+    "linux": {
+      "target": [
+        "AppImage"
+      ]
+    }
+  },
+  "devDependencies": {
+    "@types/node": "^22.0.0",
+    "@vitest/coverage-v8": "^3.1.0",
+    "electron": "^35.0.0",
+    "electron-builder": "^26.0.0",
+    "tsx": "^4.19.0",
+    "typescript": "^5.7.0",
+    "vitest": "^3.1.0"
+  }
+}

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

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+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const mocks = vi.hoisted(() => {
+  const browserWindowInstance = {
+    loadURL: vi.fn(),
+    on: vi.fn(),
+    show: vi.fn(),
+    focus: vi.fn(),
+    hide: vi.fn(),
+  };
+
+  const BrowserWindow = vi.fn(() => browserWindowInstance) as unknown as {
+    (...args: unknown[]): typeof browserWindowInstance;
+    getAllWindows: () => unknown[];
+  };
+  BrowserWindow.getAllWindows = vi.fn(() => []);
+
+  const app = {
+    whenReady: vi.fn(() => Promise.resolve()),
+    getVersion: vi.fn(() => "0.1.0"),
+    quit: vi.fn(),
+    on: vi.fn(),
+  };
+
+  const ipcMain = {
+    handle: vi.fn(),
+    on: vi.fn(),
+  };
+
+  const trayInstance = {
+    setToolTip: vi.fn(),
+    setContextMenu: vi.fn(),
+  };
+
+  const Tray = vi.fn(() => trayInstance);
+  const Menu = { buildFromTemplate: vi.fn(() => ({ id: "mock-menu" })) };
+  const nativeImage = { createEmpty: vi.fn(() => ({ id: "mock-image" })) };
+
+  return {
+    app,
+    BrowserWindow,
+    ipcMain,
+    trayInstance,
+    Tray,
+    Menu,
+    nativeImage,
+    browserWindowInstance,
+  };
+});
+
+vi.mock("electron", () => ({
+  app: mocks.app,
+  BrowserWindow: mocks.BrowserWindow,
+  ipcMain: mocks.ipcMain,
+  Tray: mocks.Tray,
+  Menu: mocks.Menu,
+  nativeImage: mocks.nativeImage,
+}));
+
+async function importMainModule() {
+  return import("../main.ts");
+}
+
+describe("main process", () => {
+  const originalDashboardUrl = process.env.FUSION_DASHBOARD_URL;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.resetModules();
+    if (originalDashboardUrl === undefined) {
+      delete process.env.FUSION_DASHBOARD_URL;
+    } else {
+      process.env.FUSION_DASHBOARD_URL = originalDashboardUrl;
+    }
+  });
+
+  it("DASHBOARD_URL defaults to localhost:4040", async () => {
+    delete process.env.FUSION_DASHBOARD_URL;
+
+    const { DASHBOARD_URL } = await importMainModule();
+
+    expect(DASHBOARD_URL).toBe("http://localhost:4040");
+  });
+
+  it("DASHBOARD_URL uses env override", async () => {
+    process.env.FUSION_DASHBOARD_URL = "http://localhost:5050";
+
+    const { DASHBOARD_URL } = await importMainModule();
+
+    expect(DASHBOARD_URL).toBe("http://localhost:5050");
+  });
+
+  it("createMainWindow creates BrowserWindow with secure preferences", async () => {
+    const { createMainWindow } = await importMainModule();
+
+    createMainWindow();
+
+    expect(mocks.BrowserWindow).toHaveBeenCalledTimes(1);
+    const [options] = mocks.BrowserWindow.mock.calls[0] as [
+      {
+        webPreferences: {
+          contextIsolation: boolean;
+          nodeIntegration: boolean;
+          preload: string;
+        };
+      },
+    ];
+
+    expect(options.webPreferences.contextIsolation).toBe(true);
+    expect(options.webPreferences.nodeIntegration).toBe(false);
+    expect(options.webPreferences.preload).toContain("preload.ts");
+  });
+
+  it("createMainWindow loads the dashboard URL", async () => {
+    const { createMainWindow, DASHBOARD_URL } = await importMainModule();
+
+    createMainWindow();
+
+    expect(mocks.browserWindowInstance.loadURL).toHaveBeenCalledWith(DASHBOARD_URL);
+  });
+
+  it("registerIpcHandlers registers app:get-version via handle", async () => {
+    const { registerIpcHandlers } = await importMainModule();
+
+    registerIpcHandlers();
+
+    expect(mocks.ipcMain.handle).toHaveBeenCalledWith(
+      "app:get-version",
+      expect.any(Function),
+    );
+  });
+
+  it("registerIpcHandlers registers app:quit via on", async () => {
+    const { registerIpcHandlers } = await importMainModule();
+
+    registerIpcHandlers();
+
+    expect(mocks.ipcMain.on).toHaveBeenCalledWith("app:quit", expect.any(Function));
+  });
+
+  it("importing main does not auto-start", async () => {
+    await importMainModule();
+
+    expect(mocks.app.whenReady).not.toHaveBeenCalled();
+  });
+
+  it("setupTray creates tray menu and hides window on close", async () => {
+    const { setupTray } = await importMainModule();
+
+    setupTray(mocks.browserWindowInstance as never);
+
+    expect(mocks.Tray).toHaveBeenCalledTimes(1);
+    expect(mocks.nativeImage.createEmpty).toHaveBeenCalledTimes(1);
+    expect(mocks.trayInstance.setToolTip).toHaveBeenCalledWith("Fusion");
+    expect(mocks.Menu.buildFromTemplate).toHaveBeenCalledTimes(1);
+
+    const closeCall = mocks.browserWindowInstance.on.mock.calls.find(
+      (call) => call[0] === "close",
+    );
+    expect(closeCall).toBeDefined();
+
+    const closeHandler = closeCall?.[1] as (event: { preventDefault: () => void }) => void;
+    const event = { preventDefault: vi.fn() };
+
+    closeHandler(event);
+
+    expect(event.preventDefault).toHaveBeenCalledTimes(1);
+    expect(mocks.browserWindowInstance.hide).toHaveBeenCalledTimes(1);
+  });
+});

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),
+    );
+  });
+});

packages/desktop/src/index.ts

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+export {
+  createMainWindow,
+  setupTray,
+  registerIpcHandlers,
+  DASHBOARD_URL,
+  run,
+} from "./main.js";
+
+export type { FusionDesktopAPI } from "./preload.js";

packages/desktop/src/main.ts

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+import { join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+import { app, BrowserWindow, Menu, Tray, ipcMain, nativeImage } from "electron";
+
+export const DASHBOARD_URL = process.env.FUSION_DASHBOARD_URL || "http://localhost:4040";
+
+export function createMainWindow(): BrowserWindow {
+  const mainWindow = new BrowserWindow({
+    width: 1280,
+    height: 900,
+    title: "Fusion",
+    webPreferences: {
+      preload: join(import.meta.dirname, "preload.ts"),
+      contextIsolation: true,
+      nodeIntegration: false,
+    },
+  });
+
+  void mainWindow.loadURL(DASHBOARD_URL);
+  return mainWindow;
+}
+
+export function setupTray(mainWindow: BrowserWindow): Tray {
+  const tray = new Tray(nativeImage.createEmpty());
+  tray.setToolTip("Fusion");
+
+  const menu = Menu.buildFromTemplate([
+    {
+      label: "Show Fusion",
+      click: () => {
+        mainWindow.show();
+        mainWindow.focus();
+      },
+    },
+    {
+      label: "Quit",
+      click: () => {
+        app.quit();
+      },
+    },
+  ]);
+
+  tray.setContextMenu(menu);
+
+  mainWindow.on("close", (event) => {
+    event.preventDefault();
+    mainWindow.hide();
+  });
+
+  return tray;
+}
+
+export function registerIpcHandlers(): void {
+  ipcMain.handle("app:get-version", () => app.getVersion());
+  ipcMain.on("app:quit", () => app.quit());
+}
+
+export function run(): void {
+  let tray: Tray | undefined;
+
+  app.whenReady().then(() => {
+    const mainWindow = createMainWindow();
+    tray = setupTray(mainWindow);
+    registerIpcHandlers();
+  });
+
+  app.on("window-all-closed", () => {
+    if (process.platform !== "darwin") {
+      // Keep app alive in tray on non-macOS platforms.
+      return;
+    }
+  });
+
+  app.on("activate", () => {
+    if (BrowserWindow.getAllWindows().length === 0) {
+      const mainWindow = createMainWindow();
+      tray = tray ?? setupTray(mainWindow);
+    }
+  });
+}
+
+const modulePath = fileURLToPath(import.meta.url);
+if (process.argv[1] && resolve(process.argv[1]) === modulePath) {
+  run();
+}

packages/desktop/src/preload.d.ts

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+export interface FusionDesktopAPI {
+  getAppVersion(): Promise<string>;
+  quit(): void;
+  onDashboardReady(callback: () => void): () => void;
+}
+
+declare global {
+  interface Window {
+    fusionDesktop: FusionDesktopAPI;
+  }
+}

packages/desktop/src/preload.ts

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+import { contextBridge, ipcRenderer } from "electron";
+
+export interface FusionDesktopAPI {
+  getAppVersion(): Promise<string>;
+  quit(): void;
+  onDashboardReady(callback: () => void): () => void;
+}
+
+const api: FusionDesktopAPI = {
+  getAppVersion(): Promise<string> {
+    return ipcRenderer.invoke("app:get-version");
+  },
+  quit(): void {
+    ipcRenderer.send("app:quit");
+  },
+  onDashboardReady(callback: () => void): () => void {
+    const listener = () => callback();
+    ipcRenderer.on("dashboard:ready", listener);
+    return () => {
+      ipcRenderer.removeListener("dashboard:ready", listener);
+    };
+  },
+};
+
+contextBridge.exposeInMainWorld("fusionDesktop", api);

packages/desktop/src/renderer.ts

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+// This file is a placeholder. The Electron shell currently connects to the
+// Fusion dashboard server via URL. Future tasks will embed the dashboard's
+// built client assets here.

packages/desktop/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src",
+    "types": ["node"]
+  },
+  "include": ["src/**/*"],
+  "exclude": ["src/**/*.test.ts", "src/**/__tests__/**/*"]
+}

packages/desktop/vitest.config.ts

@@ -0,0 +1,13 @@
+import { defineConfig } from "vitest/config";
+
+const maxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? "16", 10);
+
+export default defineConfig({
+  test: {
+    include: ["src/**/*.test.ts"],
+    maxWorkers,
+    fileParallelism: true,
+    pool: "threads",
+    passWithNoTests: true,
+  },
+});