Add a "Map settings" popover (gear button) on the Overview Live map with a
"Show live air traffic" switch, backed by a new client-side showAirTraffic
preference (default on, persisted in localStorage and synced like other
prefs). When off, the aircraft overlay clears and polling pauses to save
OpenSky credits; the count badge and caption reflect the hidden state.
Toggling on re-fetches immediately.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
