feat: add custom icon picker to node create/edit modal

- Add 65+ icons across 7 categories (Infrastructure, Media, Monitoring,
  Storage, Security, Automation, Dev & Containers, Communications) covering
  popular self-hosted apps: Home Assistant, Jellyfin, Plex, Grafana, Portainer,
  Pi-hole, Vaultwarden, Gitea, Nextcloud, Node-RED, Frigate, etc.
- New nodeIcons.ts utility with ICON_REGISTRY, ICON_MAP and resolveNodeIcon()
- Inline icon picker in NodeModal: collapsible panel with search + grid grouped
  by category; click to select, click again or Reset to revert to type default
- BaseNode uses resolveNodeIcon() so custom icon renders live on canvas
- Add custom_icon field to NodeData type, NodeBase/NodeUpdate schemas, Node ORM
  model, and database.py idempotent ALTER TABLE migration

frontend/src/types/index.ts

@@ -42,6 +42,7 @@ export interface NodeData extends Record<string, unknown> {
   parent_id?: string
   container_mode?: boolean
   custom_colors?: { border?: string; background?: string; icon?: string }
+  custom_icon?: string
 }
 
 export type EdgePathStyle = 'bezier' | 'smooth'