Set Brand Kit

Author/replace the Global Brand Kit and apply it to the Elementor active kit (global colors/fonts/type scale). CHANGES EVERY PAGE — requires confirm=true; dry_run=true returns the computed kit settings without writing.

Safety: this endpoint deliberately does NOT go through _assert_editable_page (which blocks all kit writes). It is safe because the plugin resolves the active kit id server-side and only ever writes _elementor_page_settings — the agent cannot redirect it at an arbitrary post.