desktop v0.1.20 + web: notify on download complete (was silent) (batch135)

Suppressing the save dialog made downloads completely silent — "nothing happened" even though
the file saved to Downloads. Now on download 'done': (1) tell the web UI → branded toast
"Saved X to your Downloads folder" when the window is focused; (2) native OS notification
(click → reveal in Explorer) when the app is minimized/in the tray, so it's never double-noticed;
(3) a failure notice. preload exposes onDownloadDone; home.html shows the toast.

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

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "biz-connect-desktop",
-  "version": "0.1.19",
+  "version": "0.1.20",
   "description": "Biz Connect technician desktop client — loads the Connect web UI with native screen capture",
   "author": {
     "name": "BizGaze",