feat(nodes): add firewall node type

New 'firewall' NodeType with Flame icon. Wired through node registry,
icon registry, all six themes (red-tinted accent per palette), NodeModal
Hardware group, and CustomStyleModal editable types.

frontend/src/components/modals/NodeModal.tsx

@@ -11,7 +11,7 @@ import { resolveNodeColors } from '@/utils/nodeColors'
 import { ICON_REGISTRY, ICON_CATEGORIES, NODE_TYPE_DEFAULT_ICONS } from '@/utils/nodeIcons'
 
 const NODE_TYPE_GROUPS: { label: string; types: NodeType[] }[] = [
-  { label: 'Hardware',       types: ['isp', 'router', 'switch', 'server', 'nas', 'ap', 'printer'] },
+  { label: 'Hardware',       types: ['isp', 'router', 'firewall', 'switch', 'server', 'nas', 'ap', 'printer'] },
   { label: 'Virtualization', types: ['proxmox', 'vm', 'lxc', 'docker_host', 'docker_container'] },
   { label: 'IoT',            types: ['iot', 'camera', 'cpl'] },
   { label: 'Generic',        types: ['computer', 'generic', 'groupRect'] },