feat(desktop 0.1.5): show version + Check for updates in Settings (#12)

- Settings now shows 'Biz Connect for Desktop · Version x.y.z' with a Check for
  updates button (desktop only, feature-detected).
- preload exposes checkForUpdates(); main.js adds the check-updates IPC (returns
  available/current/dev/error) and, when a build finishes downloading, shows a
  Restart now / Later dialog instead of only the silent on-next-launch install.
- desktop version bumped 0.1.4 -> 0.1.5.

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

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "biz-connect-desktop",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "description": "Biz Connect technician desktop client — loads the Connect web UI with native screen capture",
   "author": {
     "name": "BizGaze",