Fix native USB show uploads

[stevenmunn312-tech]

Summary

• forward the Board node's native USB routing and physical flash size through show uploads
• preserve the same target for card-reader player fit checks
• cover the frontend multipart contract and backend fbuild handoff with regression tests

Verification

• npm test (2,436 passed, 11 skipped)
• npm run lint
• npm run build
• python -m pytest backend/tests/test_show_upload.py backend/tests/test_engine.py -q (77 passed)