Track individual parts/objects separately from print plates in projects.
Useful for multi-part builds like Voron where 25 plates produce 150 parts.
Backend:
- Add target_parts_count field to Project model
- Calculate parts_progress_percent and remaining_parts in stats
- Auto-detect quantity from 3MF printable objects when archiving
- Sum archive quantities for completed_count (parts)
- Use archive_count for plates progress
Frontend:
- Add "Target Parts" input in project create/edit modal
- Show separate progress bars for plates vs parts
- Stats footer displays both plates and parts count
- Header badge shows parts progress when target set
Scripts:
- Add update_archive_quantities.py to migrate existing archives
Tests:
- Add 5 integration tests for parts tracking
- Add 3 unit tests for 3MF object extraction
Closes#85
