This commit adds the complete v2/v3 stock management functionality:
- add new stock-specific item statuses and update the status visual map
- implement full stock purchase API suite (create, list, retrieve, send, receive)
- reorganize sidebar navigation to group stock-related pages together
- add stock item CRUD APIs, status utilities, low stock filtering, and surplus booking logic
- add database migration for the `stock_items.target_qty` field
- update type definitions and add comprehensive tests for all new stock components
