Adds push_dag() and get_dag() to SiloClient:
- push_dag: PUT /api/items/{pn}/dag with nodes + edges
- get_dag: GET /api/items/{pn}/dag with optional revision filter
Closes#215
Replace 2-color Base→Crust gradient with 3-color Overlay0→Mantle→Overlay0
gradient in both KindredCreate preference pack and CatppuccinMocha theme.
Enables UseBackgroundColorMid for the 3-color effect.