Let Dex Copilot export large contact lists as multiple CSV files

Allow Dex Copilot to export a user’s contacts to downloadable CSV files, including large contact sets.

Desired outcome:

• Let the user choose the number or maximum size of files.
• Preview record counts before export.
• Provide separate downloadable CSV files.

Current gap: Contact export exists in Dex, but Copilot cannot start the export or split the result into multiple files.