feat(desktop): add React DevTools extension for Electron dev mode

Downloads and loads React DevTools Chrome extension using Electron 41's
session.extensions API directly, avoiding the deprecated session.loadExtension
used by electron-devtools-installer. Extension is cached in userData after
first download. Only loaded when !app.isPackaged.

packages/desktop/src/vendor.d.ts

@@ -0,0 +1,4 @@
+declare module "unzip-crx-3" {
+  function unzipCrx(crxPath: string, outputDir: string): Promise<void>;
+  export default unzipCrx;
+}