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/preload.js

@@ -22,4 +22,7 @@ contextBridge.exposeInMainWorld('bizConnectNative', Object.freeze({
   // Native Windows toast with an inline reply box. Resolves to {text} (replied), {open} (clicked)
   // or null. Lets the user reply to a chat straight from the notification.
   replyNotify: (payload) => ipcRenderer.invoke('reply-notification', payload),
+  // Manual "Check for updates" from Settings. Resolves {status:'available'|'current'|'dev'|'error', version?}.
+  // On 'available' the shell downloads in the background and prompts to restart when ready.
+  checkForUpdates: () => ipcRenderer.invoke('check-updates'),
 }));