Add CSV import and export for leads

[imperator-clawdius]

LaunchDesk should make it easy for non-technical users to bring in a small prospect list and export their pipeline.\n\nSuggested scope:\n- Add a CSV export button for leads.\n- Add a CSV import flow for columns like company, contact, niche, stage, value, nextAction, notes.\n- Validate rows before saving.\n- Keep the JSON API stable.\n\nVerification:\n- npm test\n- npm run smoke\n- Manual import/export with a tiny sample CSV