fix: ShieldAlert still referenced in CATEGORY_DEFAULTS after icon swap

Removed ShieldAlert from imports when swapping firewall icon to BrickWallFire
but left it in CATEGORY_DEFAULTS — runtime crash, device toolbar empty.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/network/nodes/deviceRegistry.ts

@@ -79,7 +79,7 @@ const CATEGORY_DEFAULTS: Record<string, DeviceRenderConfig> = {
   'cloud':          { icon: Cloud,       color: CLOUD_COLOR },
   'endpoint':       { icon: Monitor,     color: ENDPOINT_COLOR },
   'infrastructure': { icon: PlugZap,     color: INFRA_COLOR },
-  'security':       { icon: ShieldAlert, color: SECURITY_COLOR },
+  'security':       { icon: BrickWallFire, color: SECURITY_COLOR },
 }
 
 const FALLBACK: DeviceRenderConfig = { icon: Cpu, color: INFRA_COLOR }