Add Data Export Functionality

[pyang4s]

Description:
Create a script and notebook functionality to export transaction data to CSV/Excel format.

Acceptance Criteria:

• Create Python script scripts/export_data.py
• Support CSV and Excel export formats
• Allow filtering by date range, person, category
• Add notebook cell demonstrating export usage
• Include all transactions with joined category/person names
• Create exports/ directory and add to .gitignore
• Document usage in README

Estimated Effort: 2-3 hours