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>
Centralize all API call logic (fetchBoxInfo, saveBoxRecord,
updateBoxRecord, deleteBoxRecord) into BoxingApiActions class with
unified error categorization (network, duplicate, missingApiUrl).
Add helper methods to reduce repetitive error handling in boxing_page.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>