Use grey dot when ARP scanner is waiting for next run

Amber implied a problem when the scanner is simply idle between runs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/widgets/arp_scanner_card.dart

@@ -31,7 +31,7 @@ class ArpScannerCard extends StatelessWidget {
         double.infinity,
       );
       return (
-        color: Colors.amber,
+        color: Colors.grey,
         label: 'Waiting for next run',
         sublabels: ['Next run in ${formatSeconds(remaining)}'],
       );