Add Set<number>-based selection state to ScholarsPage with checkboxes
in both table and card views, select-all toggle, and stale-key pruning.
Backend: POST /scholars/bulk-delete, POST /scholars/bulk-toggle, and
GET /scholars/export?ids= filter. Service functions use user-scoped
queries. Structured logging for bulk operations.
Frontend: useScholarBulkActions composable extracts selection and bulk
action logic. AppSelect-based action dropdown with dynamic options.
Delete prompts window.confirm; enable/disable applies immediately.
Export downloads filtered JSON.
Includes integration tests for all bulk endpoints and frontend unit
tests for selection toggle, select-all, and stale pruning.
