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/main.ts

@@ -0,0 +1,85 @@
+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();
+}