fix(desktop): custom screen picker, single restart prompt, reliable call Join (0.1.8/batch61)

- Custom branded screen/window picker (picker.html) — useSystemPicker was
  silently no-op on Win11 and auto-shared the primary display with no choice.
- Use checkForUpdates (not ...AndNotify): drops electron-updater's own native
  "Update ready" toast that duplicated our in-app banner (two restart prompts).
- Call notification DP: wait up to 2.5s for the caller photo on persistent
  (call) toasts instead of 600ms so the DP actually shows.
- Re-surface Join/Decline invite when opening a chat with an active incoming
  call, so a call-notification click always lands on a joinable call.

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

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "biz-connect-desktop",
-  "version": "0.1.7",
+  "version": "0.1.8",
   "description": "Biz Connect technician desktop client — loads the Connect web UI with native screen capture",
   "author": {
     "name": "BizGaze",