Replace direct row rendering with @tanstack/vue-virtual. The table
now only renders visible rows (~20) via padding <tr> placeholders,
eliminating the rendering bottleneck when displaying 100+ rows with
heavy cell components.
Key changes:
- DataTable.vue: integrate useVirtualizer (always-on), virtual row
template with measureElement for variable row heights, defineExpose
virtualizer/sortedData for external access, overflow-y/flex CSS
- useSwipeSelect.ts: dual-mode support via optional
SwipeSelectVirtualContext — data-driven row index lookup and
selection range when virtualizer is present, original DOM-based
path preserved for callers that don't pass virtualContext
- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods
