Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.
Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
user_group_rate_multipliers table (same table as user-side rates)
Frontend:
- New GroupRateMultipliersModal component with:
- User search and add with email autocomplete
- Editable rate column with local edit mode (cancel/save)
- Batch adjust: multiply all rates by a factor
- Clear all (local operation, requires save to persist)
- Pagination (10/20/50 per page)
- Platform icon with brand colors in group info bar
- Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints
- 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
