Customer Avatar Upload:
- Add /account/avatar endpoint for upload/delete
- Add /account/avatar-settings endpoint for settings
- Update AccountDetails.tsx with avatar upload UI
- Support base64 image upload with validation
Product Downloadable Files:
- Create DownloadsTab component for file management
- Add downloads state to ProductFormTabbed
- Show Downloads tab when 'downloadable' is checked
- Support file name, URL, download limit, and expiry
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