Add complete end-to-end QuickBooks invoicing workflow for the invoicing page:
- Add `jspdf` dependency for client-side PDF generation
- Implement updated item status handling with fallback for new invoice statuses
- Add server-side proxy API for secure QuickBooks customer lookup
- Build utilities to combine QuickBooks invoices and picking slips into a single PDF
- Add invoicing view models for row grouping, payload construction, and status checks
- Add full test coverage for all new components, APIs, and utilities
- Include feature design documentation and sample n8n workflow config
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