fix(payment): wire balance wallet end-to-end when pay_balance_s is on

Opening 余额功能 alone now enables cash-wallet pay (unless 余额支付 is explicitly off). Drop points-as-balance fallback, stop fake method injection, expose real wallet in cashiers, and honor auto_to_balance on recharge fulfill.

assets/admin/src/schema/sections/payment/gateway.js

@@ -12,7 +12,7 @@ export const gatewaySchema = {
     payment_wechat_enabled: false,
     payment_alipay_enabled: false,
     payment_paypal_enabled: false,
-    payment_balance_enabled: false,
+    payment_balance_enabled: true,
     payment_card_pass_enabled: false,
     payment_xunhupay_enabled: false,
     payment_xhpay_enabled: false,
@@ -73,7 +73,7 @@ export const gatewaySchema = {
         { key: 'payment_alipay_enabled', label: '支付宝', type: 'switch', default: false },
         { key: 'payment_paypal_enabled', label: 'PayPal', type: 'switch', default: false },
         { key: 'payment_stripe_enabled', label: 'Stripe', type: 'switch', default: false },
-        { key: 'payment_balance_enabled', label: '余额支付', type: 'switch', default: false },
+        { key: 'payment_balance_enabled', label: '余额支付', type: 'switch', default: true, help: '需同时在「余额充值」启用余额功能。关闭后用户中心仍可有钱包，但结账不可用余额支付。' },
         { key: 'payment_card_pass_enabled', label: '卡密充值', type: 'switch', default: false },
         { key: 'payment_xunhupay_enabled', label: '虎皮椒', type: 'switch', default: false },
         { key: 'payment_xhpay_enabled', label: '小火付', type: 'switch', default: false },