Implement Xledger Exporter service

Structure the service as XledgerExporter with injected dependencies: ChartOfAccountsMapper, CsvJsonFileGenerator, ExportRunRepository, SupabaseStorageClient. The transformation pipeline should be: (1) map each ApprovedClaim to an XledgerJournalEntry using XledgerRecordMapper (pure, synchronous, easily testable); (2) validate the full batch (sum debits == sum credits, no blank required fields); (3) serialize to CSV via CsvJsonFileGenerator; (4) upload to Supabase Storage; (5) create export_runs record; (6) return ExportResult. Define XledgerJournalEntry as an immutable Dart class with all 7 required fields typed (e.g., amount as int in øre to avoid floating-point rounding). Use Dart's intl DateFormat for voucher_date formatting.

For the double-entry row pair: debit row has debit_amount set and credit_amount = 0; credit row is the inverse. Wrap the entire export in a try/catch that writes a 'failed' export_run record before rethrowing — so the failure is always recorded. Edge Function deployment is out of scope for this task; the Flutter service should call the Edge Function endpoint rather than calling Supabase Storage directly with a service key.

Unit tests (flutter_test with mocked dependencies): test XledgerRecordMapper.toXledgerRows(claim) for each claim type (mileage, toll, parking, public transit); assert debit/credit balance; assert field truncation at 50 chars; assert missing account code moves claim to unresolvable list. Integration test: invoke full export() with a list of 5 mock ApprovedClaims against a test Supabase Storage bucket; verify file created, signed URL returned, export_runs record created with status 'completed'. Test file: test/services/xledger_exporter_test.dart. Mock ChartOfAccountsMapper and CsvJsonFileGenerator in unit tests.

Cover validation failure path: pass a claim with negative amount, verify ValidationFailureResult returned.