station: Identify devices by MAC OUI vendor lookup

Embed the IEEE OUI registry (~1MB pre-processed text file) and resolve
the vendor for every station MAC. Locally administered MACs (U/L bit
set, used by iOS/Android private addresses and virtual interfaces) are
skipped so we don't return spurious matches against randomized prefixes.

The vendor name shows up in the device card as a secondary line, and
falls back to the title position when no DHCP hostname is available —
"Apple" with the IP and MAC is far more useful than "Sans nom".

The lookup table loads lazily (sync.Once) on the first call so the
~40k-entry parse only runs when the station discovery code is exercised.

internal/station/station.go

@@ -68,6 +68,7 @@ func toConnectedDevice(s backend.Station) models.ConnectedDevice {
 		Type:        s.Type,
 		MAC:         s.MAC,
 		IP:          s.IP,
+		Vendor:      s.Vendor,
 		SignalDbm:   s.Signal,
 		RxBytes:     s.RxBytes,
 		TxBytes:     s.TxBytes,