Reset QR server state after shutdown

[cubrift]

Problem

stopQRHost() in QRHost.js closes the HTTP server but never sets the server variable back to null. During a rapid reconnect, a new QR server can be requested while the previous server is still closing, which may lead to a port conflict or stale server state.

Expected behavior

After the QR host stops, the module should be ready to start a fresh server cleanly.

Acceptance criteria

• The server reference is cleared when shutdown begins or completes.
• A new QR session can start after a previous session stops without reusing stale state.
• currentQrData remains cleared after shutdown.
• Repeated calls to stopQRHost() are harmless.

[cubrift]

There will never be a moment where another QR server needs to be made again.