Add CSV Export to Lovable App

Add CSV export functionality to my Lovable app.

REQUIREMENTS:
1. Add an "Export CSV" button on data table pages.
2. Open a modal where users can select which columns to include in the export.
3. Add date range filter (start date, end date) to limit exported rows.
4. Fetch the filtered data from Supabase and convert to CSV format.
5. Use UTF-8 BOM encoding for Excel compatibility with special characters.
6. Handle commas and quotes in data by properly escaping values.
7. Generate the CSV client-side and trigger a browser download with a filename including the current date (e.g., users-export-2024-03-15.csv).
8. Show a loading indicator during export for large datasets.
9. Limit export to 10,000 rows maximum to prevent browser memory issues.

DATABASE:
- No new tables. Queries existing tables with selected columns.

SECURITY:
- Respect RLS — only export data the user has permission to view.
- Log exports in an audit table if handling sensitive data.