feat(desktop): packaging + self-hosted auto-update (electron-builder/updater)

- package.json build config: NSIS installer, app icon, generic publish provider →
  https://remote.bizgaze.com/downloads/ (self-hosted update feed).
- main.js: electron-updater checks the feed on launch + every 6h, downloads in the
  background, installs on restart. Active only in packaged builds.
- build/icon.ico app icon; PACKAGING.md documents build/release/signing.
- Key design: web/UI changes reach installed apps instantly (they load the live server);
  only native shell changes need an auto-updated build.

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

.gitignore

@@ -33,6 +33,10 @@ out/
 mobile/android/
 mobile/ios/
 
+# Keep the desktop app icon (a build RESOURCE), even though the global build/ rule ignores it
+!desktop/build/
+!desktop/build/**
+
 # Runtime media (created at startup by config.js)
 server/recordings/
 server/transcripts/