fix(desktop): set productName so Electron uses Fusion as app name

Without productName, Electron's app.getName() returns the package
name "@fusion/desktop", which macOS uses for the user-data dir and
some app surfaces. Setting productName=Fusion aligns runtime naming
with the electron-builder productName and the Info.plist bundle name.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/desktop/package.json

@@ -1,5 +1,6 @@
 {
   "name": "@fusion/desktop",
+  "productName": "Fusion",
   "version": "0.31.0",
   "license": "MIT",
   "description": "Fusion desktop: Electron wrapper around the Fusion dashboard for macOS, Windows, and Linux.",