feat: add Apple IAP (apple_iap) payment method support

Add apple_iap across all payment surfaces: method label, icon (Apple
logo SVG), admin settings tree (APPLE_IAP category), and all four
locale files (en, ru, fa, zh) with payment description + tree label.

src/locales/zh.json

@@ -496,6 +496,9 @@
       },
       "severpay": {
         "description": "通过SeverPay支付"
+      },
+      "apple_iap": {
+        "description": "通过Apple应用内购买支付"
       }
     },
     "transactionHistory": "交易记录",
@@ -1659,6 +1662,7 @@
         "payments_paypear": "PayPear",
         "payments_rollypay": "RollyPay",
         "payments_aurapay": "AuraPay",
+        "payments_apple_iap": "Apple IAP",
         "subs_core": "核心",
         "subs_trial": "试用",
         "subs_pricing": "定价",