(SP: 1)[SHOP] clarify payment env example comments for provider gating and defaults

Author: liudmylasovetovs

frontend/.env.example

@@ -65,7 +65,7 @@ STRIPE_MODE=
 # Toggle Stripe payments for Shop checkout flows.
 STRIPE_PAYMENTS_ENABLED=
 
-# Required when Stripe payments are enabled.
+# Required when PAYMENTS_ENABLED enables Shop payments and Stripe is used as an active payment rail.
 # In production-like runtime, invalid or placeholder config must fail closed.
 STRIPE_SECRET_KEY=
 STRIPE_WEBHOOK_SECRET=
@@ -75,9 +75,10 @@ STRIPE_WEBHOOK_SECRET=
 MONO_API_BASE=
 
 # Optional invoice timeout override in milliseconds.
+# Default fallback: 8000 in production, 12000 outside production.
 MONO_INVOICE_TIMEOUT_MS=
 
-# Required when Monobank checkout/webhooks are enabled.
+# Required when PAYMENTS_ENABLED enables Shop payments and Monobank is used as an active payment rail.
 # In production-like runtime, invalid or placeholder config must fail closed.
 MONO_MERCHANT_TOKEN=
 MONO_PUBLIC_KEY=