1. Add allow_custom_avatar toggle to Customer Settings
2. Implement coupon apply/remove in Cart and Checkout pages
3. Update Cart interface with coupons array and discount_total
4. Implement Downloads page to fetch from /account/downloads API
- Created Newsletter/index.tsx as tabs container
- Extracted Newsletter/Subscribers.tsx (from old Newsletter.tsx)
- Moved Campaigns to Newsletter/Campaigns.tsx
- Updated App.tsx routes (campaigns now under newsletter)
- Removed separate Campaigns card from Marketing index
- Follows Customer Notifications tab pattern for consistency