fix(desktop 0.1.6): fast + persistent native notifications + screen picker

- Desktop notifications were slow (~15s), vanished in ~1s, and clicks did nothing:
  * don't block the toast on the avatar download (race a 600ms cap) → shows instantly
  * keep a strong reference to each Notification (Electron GC'd them → premature close
    + dead click)
  * call invites use timeoutType:'never' + a 45s window so they stay until clicked/ended;
    web marks call notifications persistent.
- #9: enable the OS screen/window PICKER (useSystemPicker) so users choose what to share
  (a single window avoids the whole-screen mirror); falls back to primary display.
- desktop 0.1.5 -> 0.1.6.

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

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "biz-connect-desktop",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "description": "Biz Connect technician desktop client — loads the Connect web UI with native screen capture",
   "author": {
     "name": "BizGaze",