Mobile POS beta 18 — connection and server switching fixes

Mobile POS beta 18 — cloud connection and refresh fix

• Cloud authorization keeps the approved internet server instead of silently pairing to a nearby Wi-Fi-only till.
• Background sync no longer displays the pull-to-refresh spinner. Local-only offline connections explain the Wi-Fi requirement.
• Change server stays available while an old address is retrying. Opening its confirmation pauses further retries; disconnect waits for the current operation to finish and retains protections for pending sales, unfinished baskets and printing.
• Existing server pairing and unsynced sales are preserved. This update does not automatically migrate a device already paired to an old local IP.

For an existing local pairing whose computer IP changed: More → Connection & sync → Server settings → Change server. Finish or sync pending work before switching. Enter the computer's current address, or choose cloud sign-in for internet access.

Validation: 106 unit tests, typecheck, attribution, formatting and all 44 browser regression tests passed. Native Android compilation and signing compatibility are checked by the linked build. Physical-device reconnect testing remains a pilot check; Google Play submission stays on hold.

Install over the existing GitHub pilot APK. Do not uninstall or clear app data with pending sales.