Feature: Generate a PDF reading report

[texasbe2trill]

Summary

Generate a printable PDF report summarizing a user's reading data — library stats, reading streaks, top highlights, activity charts, and insight summaries. Useful for personal journaling or sharing.

Requirements

• Create services/export_pdf.py
• Include: library overview, per-book summaries, top highlights, reading streak stats, activity charts
• Render charts as static images (Plotly supports write_image)
• Register in CLI (konotes export <file> -f pdf)
• Add as optional dependency (pip install '.[pdf]')
• Add tests

Considerations

• Libraries to consider: weasyprint, reportlab, or fpdf2
• Could reuse the HTML export as an intermediate step and convert to PDF
• Keep it as an optional extra to avoid heavy dependencies for users who don't need it