The shared audio queue (maxsize reduced from 80 to 20) was not flushed
when the monitor frequency changed — only when the monitor was disabled.
This caused up to 4 seconds of stale old-frequency audio to play after
clicking to tune, making click-to-tune appear non-functional.
Now flushes the queue whenever the VFO frequency changes, so audio at
the new frequency begins within ~50ms (one FFT frame).
a5452fa1b1