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

@@ -0,0 +1,32 @@
+# @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`)