refactor: extract boxing page into modular components
Split the monolithic boxing_page.dart into separate files: - boxing_models.dart: data models (_ManyToOnePackedItem, _Phase) - widgets/boxing_shared_widgets.dart: shared UI components - widgets/single_code_body.dart: single-code boxing UI - widgets/multi_code_body.dart: multi-code boxing UI - widgets/multi_packed_row.dart: multi-code packed row widget - widgets/single_assigned_row.dart: single-code assigned row widget Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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