cabana: fix UI hang when switching streams

[deanlee]

Switching streams could freeze the UI because the previous stream object was destroyed synchronously. Its destructor could trigger a BlockingQueuedConnection (see link below), which blocks the main thread and causes Cabana to hang:

openpilot/tools/cabana/streams/replaystream.cc

Line 62 in 8ffe3f2

replay->onSegmentsMerged = [this]() { QMetaObject::invokeMethod(this, &ReplayStream::mergeSegments, Qt::BlockingQueuedConnection); };

Using deleteLater() defers the destruction to Qt’s event loop, avoiding the blocking call and preventing the hang.

Reproduction steps:

1. Run: cabana "<route>"
2. In the File menu, choose Open Stream and open another stream.
3. The UI may hang at this point.