Silo Spreadsheet Sync for LibreOffice Calc

Bidirectional sync between LibreOffice Calc spreadsheets and the Silo
parts database. Pull project BOMs, edit in Calc, push changes back.

Features:
- Pull BOM: fetch an expanded bill of materials as a formatted sheet
- Pull Project: fetch all items tagged with a project code
- Push: sync local edits (new items, modified fields) back to the database
- Add Item wizard: guided workflow for adding new BOM entries
- PN conflict resolution: handle duplicate part numbers gracefully
- Auto project tagging: items in a working BOM are tagged with the project

Toolbar commands appear when a Calc spreadsheet is active.
Settings and API token are stored in ~/.config/silo/calc-settings.json.