Show airports with ATC online on the map (VATSIM + IVAO)

[lyestarzalt]

Reported by Jean.

Summary

The current VATSIM and IVAO overlays only render traffic. Users can't tell at a glance which airports have controllers online without opening each airport panel. Adding a small map indicator on airports with active ATC would improve situational awareness.

Behaviour

• When VATSIM is enabled: highlight airports that have at least one controller online (TWR / GND / APP / DEL / ATIS).
• When IVAO is enabled: same, once IVAO controllers are wired up — currently useIvaoQuery only handles clients.pilots, clients.atcs is unused.
• Indicator should be subtle, not overwriting the existing airport dot. Examples: a small ring, a tinted halo, or a tiny radio glyph next to the dot.

[lyestarzalt]

VATSIM half is done (4da648c). Airports with active ATC now show an indicator on the map and controller details in the airport panel. IVAO half still pending: useIvaoQuery reads clients.pilots but not clients.atcs yet.