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/hostapd/backend.go

@@ -328,6 +328,7 @@ func (b *Backend) convertStation(mac string, hs *HostapdStation) backend.Station
 		IP:          ip,
 		Hostname:    hostname,
 		Type:        backend.GuessDeviceType(hostname, mac),
+		Vendor:      backend.LookupVendor(mac),
 		Signal:      hs.Signal,
 		RxBytes:     hs.RxBytes,
 		TxBytes:     hs.TxBytes,