Add new and returning attendees report

[nbudin]

Summary

• Adds a new report showing which convention attendees are first-timers vs. returning attendees within an organization
• Implements the SQL query from Add "new and returning attendees" report #11190 as ActiveRecord in ConventionReportsPresenter
• New new_and_returning_attendees GraphQL field on ConventionReports type
• New React page at /reports/new_and_returning_attendees showing a summary count and sortable tables for each group

Test plan

• Visit /reports/new_and_returning_attendees at a convention that belongs to an organization
• Verify new attendees (only this convention in the org) appear in the "New attendees" table
• Verify returning attendees (attended other org conventions) appear in the "Returning attendees" table
• Verify the summary counts match the table row counts
• Verify the link appears in the Reports menu

Fixes #11190

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/types/convention_reports_type.rb	🟢 78.26%	🟢 80%	🟢 1.74%
app/graphql/types/new_and_returning_attendees_type.rb	🔴 0%	🟢 100%	🟢 100%
app/graphql/types/organization_attendance_count_type.rb	🔴 0%	🟢 80%	🟢 80%
app/presenters/convention_reports_presenter.rb	🔴 0%	🟠 50%	🟢 50%
test/presenters/convention_reports_presenter_test.rb	🔴 0%	🟢 100%	🟢 100%
Overall Coverage	🟢 52.25%	🟢 52.62%	🟢 0.37%

Minimum allowed coverage is 0%, this run produced 52.62%