Review Step with Validation and CSV Download

Display transformation results with validation checks and CSV download before pushing to Costpoint.

Note: - Display summary: total transactions, total journal lines, date range, transaction types included - Show validation results: unmapped accounts (if any), invalid entries, accounting balance check - Block "Push to Costpoint" button if unmapped transactions exist - Display unmapped accounts prominently with link to configuration UI to fix mappings - CSV download button: generate and download CSV for QA/audit purposes - CSV format: line-item level with Costpoint account codes, debits, credits, project codes, QB audit info - CSV columns match Costpoint import format (reference PoC for format) - Show sample of transformed data: first 10-20 journal lines in preview table - Validation errors displayed clearly with actionable guidance - Only allow proceeding to push step when all validations pass

Open questions: - Should we allow downloading CSV even when unmapped accounts exist? - Should we show transaction-level summary or only line-item details?