feat: implement accounting export for subscription transactions

[DevSolex]

Summary

Implements #390 — accounting export system for subscription transactions.

Closes #390

Changes

src/services/accountingExport.ts

• Date range filtering (dateFrom/dateTo) and transaction type filtering
• JSON format export builder
• Deferred revenue column support for GAAP compliance
• Export content stored in history for re-download

src/screens/AccountingExportScreen.tsx

• All four formats exposed in UI: CSV, JSON, QuickBooks, Xero
• History entries tappable for re-download (copies to clipboard)

backend/services/accountingExportService.ts (new)

• streamExport() — chunked streaming for large datasets across all 4 formats
• reconcile() — detects missing records, amount mismatches, type mismatches

Acceptance Criteria

• Export formats: CSV, JSON, QuickBooks, Xero
• Configurable date range and filter options
• Transaction categorization (revenue, refund, credit, fee)
• Deferred revenue calculation for GAAP compliance
• Scheduled automatic exports
• Export history with re-download

Edge Cases Handled

• Large dataset streaming via chunked output
• CSV encoding safety (NFC normalization, quote escaping)
• Reconciliation mismatch detection

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