fix: persist custom_icon on canvas save + add CCTV icon

- NodeSave schema was missing custom_icon field — icon reset to default on reload
- handleSave was not including custom_icon in the payload
- CameraNode now uses Cctv icon (clearer than Camera)
- Added 'cctv' entry to ICON_REGISTRY for the icon picker

backend/app/schemas/canvas.py

@@ -22,6 +22,7 @@ class NodeSave(BaseModel):
     parent_id: str | None = None
     container_mode: bool = False
     custom_colors: dict[str, Any] | None = None
+    custom_icon: str | None = None
     pos_x: float = 0
     pos_y: float = 0