Add Explorer page with card grid, drill-down nav, search, and pagination (Task 6)

ExplorerState dataclass tracks navigation level (kinds/nodes), breadcrumb
trail, and pagination. create_explorer_page() builds the NiceGUI UI with
kind cards (icon, count, preview, explore/details buttons), node cards
(label, subtitle, property badges, details button), detail modals with
properties table and edge lists, client-side JS search filtering, and
pagination controls. 11 state management tests included.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aksOps