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/preload.ts

@@ -0,0 +1,25 @@
+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);