_refresh_busy_banner() toggled the banner's visibility synchronously
on every poll tick and scan-queue push message, so brief blips in the
underlying queue status made it flicker on/off.
Show busy immediately, but hold the banner up for 5s of continuous
idle before clearing it, mirroring the debounce pattern already used
for XRayEye's queue-guarded toggles (_queue_idle_timer).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>