Add client-side Export CSV button to attendance history report

[nbudin]

Summary

• Adds an "Export CSV" button to the attendance history report, inline with the page title
• Entirely client-side using csv-stringify and file-saver (same pattern as mailing lists)
• CSV columns: Name, Email, Conventions (count — not the full list of convention names)
• Downloads as attendance_history.csv

Test plan

• Visit the attendance history report and click "Export CSV"
• Verify the downloaded CSV has Name, Email, and Conventions count columns
• Verify the convention count matches what's shown in the table

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 52.66%	🟢 52.65%	🔴 -0.01%

Minimum allowed coverage is 0%, this run produced 52.65%