feat: enhance data export -- server-side PDF generation and export filters

[Sagargupta16]

Summary

Enhance the existing export functionality with server-side PDF report generation and category/account filtering.

What Already Exists

• CSV export via /api/transactions/export with date range filtering
• "Export CSV" button on Transactions page
• HTML-based monthly report via /api/reports/monthly (browser print-to-PDF)

What's Missing

1. Server-Side PDF Generation

• Generate proper PDF reports (not browser print-to-PDF)
• Use a lightweight library (e.g., reportlab or weasyprint)
• Include: summary stats, category breakdown table, monthly trends
• Endpoint: GET /api/reports/generate?format=pdf&date_from=...&date_to=...

2. Export Filters

• Filter CSV export by category, account, or tag (not just date range)
• Bulk export across multiple months

3. Converted Currency in Exports

• CSV should use display_currency amounts when user has currency conversion active
• Note the exchange rate used in export metadata

Acceptance Criteria

• PDF report downloads as a proper .pdf file
• CSV export supports category and account filters
• Exported amounts respect display currency setting