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

backend/app/schemas/nodes.py

@@ -21,6 +21,7 @@ class NodeBase(BaseModel):
     parent_id: str | None = None
     container_mode: bool = False
     custom_colors: dict[str, Any] | None = None
+    custom_icon: str | None = None
 
 
 class NodeCreate(NodeBase):
@@ -43,6 +44,7 @@ class NodeUpdate(BaseModel):
     pos_y: float | None = None
     container_mode: bool | None = None
     custom_colors: dict[str, Any] | None = None
+    custom_icon: str | None = None
 
 
 class NodeResponse(NodeBase):