Implement data export (CSV/JSON) and bank import (OFX/QFX)

[larralapid]

Summary

Build the export/import pipeline: export receipts as CSV/JSON, import bank transactions from OFX/QFX files.

Context

Settings screen has stub placeholders for Export Data and Import Bank Transactions. These are critical for tax prep and bank reconciliation workflows.

Acceptance Criteria

• Export all receipts as CSV with configurable date range
• Export as JSON for backup/migration
• Import OFX/QFX bank statement files
• Parse transactions and create BankTransaction records
• Match imported transactions to existing receipts by amount/date
• Share sheet integration for export

Technical Notes

• BankTransaction model already exists but has no UI or import flow
• Settings stubs: SettingsView.swift rows for Export/Import
• OFX is XML-based — can parse with Foundation XMLParser
• Consider also supporting CSV bank exports