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/package.json

@@ -0,0 +1,50 @@
+{
+  "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"
+  }
+}