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/dhcp/backend.go

@@ -72,6 +72,7 @@ func (b *Backend) GetStations() ([]backend.Station, error) {
 				IP:          lease.IP,
 				Hostname:    lease.Hostname,
 				Type:        backend.GuessDeviceType(lease.Hostname, lease.MAC),
+				Vendor:      backend.LookupVendor(lease.MAC),
 				Signal:      0, // Not available from DHCP
 				RxBytes:     0, // Not available from DHCP
 				TxBytes:     0, // Not available from DHCP