feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

frontend/src/assets/icons/alipay.svg

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frontend/src/assets/icons/easypay.svg

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frontend/src/assets/icons/stripe.svg

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frontend/src/assets/icons/wxpay.svg

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