store compact RSSI stats and skip real-time collection

Stats.WiFi is now map[string]int8 (json "wf") holding only available
RSSI
readings; SSID and unavailable signals stay in Info.WiFi. Averages are
rounded to whole dBm.

Wi-Fi is collected only on the default 60s interval; real-time requests
reuse the last snapshot to avoid spawning osascript / dumping the BSS
cache every second.

internal/site/src/components/routes/system/charts/wifi-chart.tsx

@@ -20,7 +20,7 @@ export function WiFiChart({
 	const dataPoints = interfaces.map(([id, current]) => ({
 		label: current.s ? `${id} (${current.s})` : id,
 		color: wifiColor(id),
-		dataKey: ({ stats }: SystemStatsRecord) => stats?.wifi?.[id]?.r,
+		dataKey: ({ stats }: SystemStatsRecord) => stats?.wf?.[id],
 	}))
 	return (
 		<ChartCard