chore(mobile): bump iOS buildNumber to 6 for next TestFlight upload

Apple requires monotonically increasing build numbers. Build 5 was the
prior planned upload; bumping to 6 for the actual archive that ships
the App Store followup work (privacy manifest, in-app privacy/support
links, default-off notifications, connection-stability fixes).

Co-authored-by: Orca <help@stably.ai>

mobile/app.json

@@ -16,7 +16,7 @@
     "ios": {
       "supportsTablet": false,
       "bundleIdentifier": "com.stably.orca.mobile",
-      "buildNumber": "5",
+      "buildNumber": "6",
       "infoPlist": {
         "NSLocalNetworkUsageDescription": "Orca connects to the desktop app on your local network.",
         "NSAppTransportSecurity": {