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/arp/backend.go

@@ -59,11 +59,13 @@ func (b *Backend) GetStations() ([]backend.Station, error) {
 	for _, entry := range arpEntries {
 		// Only include entries with valid flags (2 = COMPLETE, 6 = COMPLETE|PERM)
 		if entry.Flags == 2 || entry.Flags == 6 {
+			mac := entry.HWAddress.String()
 			st := backend.Station{
-				MAC:         entry.HWAddress.String(),
+				MAC:         mac,
 				IP:          entry.IP.String(),
 				Hostname:    "", // No hostname available from ARP
-				Type:        backend.GuessDeviceType("", entry.HWAddress.String()),
+				Type:        backend.GuessDeviceType("", mac),
+				Vendor:      backend.LookupVendor(mac),
 				Signal:      0, // Not available from ARP
 				RxBytes:     0, // Not available from ARP
 				TxBytes:     0, // Not available from ARP