feat: Multiple fixes and features

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

admin-spa/src/routes/Marketing/Newsletter/index.tsx

@@ -56,7 +56,7 @@ export default function Newsletter() {
             description={__('Manage subscribers and send email campaigns')}
         >
             <Tabs value={activeTab} onValueChange={handleTabChange} className="space-y-6">
-                <TabsList className="grid w-full grid-cols-2 max-w-md">
+                <TabsList className="grid w-full max-w-md grid-cols-2">
                     <TabsTrigger value="subscribers">{__('Subscribers')}</TabsTrigger>
                     <TabsTrigger value="campaigns">{__('Campaigns')}</TabsTrigger>
                 </TabsList>