Extract large methods from _BoxingPageState into 6 part files:
- boxing_scan_part.dart: scan handling and auto-scan processing
- boxing_submit_part.dart: submit/update/delete API calls
- boxing_box_mutation_part.dart: box item mutations
- boxing_single_ops_part.dart: single-code boxing operations
- boxing_multi_ops_part.dart: multi-code boxing operations
- boxing_status_part.dart: status bar management
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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