desktop: v0.1.19 — reliable auto-update for close-to-tray users

The app closes to tray, so it kept running on the old version and only checked on a fresh launch.
Now: (1) check for updates on EVERY window show (X-to-tray then reopen, taskbar, relaunch via
second-instance) via win.on('show'), throttled 1/10min; (2) explicit autoInstallOnAppQuit=true so
a downloaded update installs on the next real quit / PC restart even if the user never clicks
"Restart now"; (3) native notification when an update finishes downloading while hidden in the
tray. The existing 6-hour background timer is unchanged and still runs regardless of window state.
Also carries the 0.1.18 downloads-to-Downloads-folder change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "biz-connect-desktop",
-  "version": "0.1.18",
+  "version": "0.1.19",
   "description": "Biz Connect technician desktop client — loads the Connect web UI with native screen capture",
   "author": {
     "name": "BizGaze",