fix: handle unlimited traffic package selection and button text

- Fix falsy check: selectedTrafficPackage !== null instead of truthy
  (gb=0 for unlimited was evaluated as false, hiding the buy button)
- Show "Buy Unlimited" instead of "Buy 0 GB" for unlimited packages
- Add buyUnlimited translation key to all 4 locales

src/locales/en.json

@@ -454,6 +454,7 @@
       "trafficUnavailable": "Traffic purchase is not available for your tariff",
       "unlimited": "Unlimited",
       "buyTrafficGb": "Buy {{gb}} GB",
+      "buyUnlimited": "Buy Unlimited",
       "manageServers": "Server management",
       "manageServersTitle": "Server management"
     },

src/locales/fa.json

@@ -328,6 +328,7 @@
       "trafficUnavailable": "خرید ترافیک برای اشتراک شما در دسترس نیست",
       "unlimited": "نامحدود",
       "buyTrafficGb": "خرید {{gb}} GB",
+      "buyUnlimited": "خرید نامحدود",
       "manageServers": "مدیریت سرورها",
       "manageServersTitle": "مدیریت سرورها"
     },

src/locales/ru.json

@@ -481,6 +481,7 @@
       "trafficUnavailable": "Докупка трафика недоступна для вашего тарифа",
       "unlimited": "Безлимит",
       "buyTrafficGb": "Купить {{gb}} ГБ",
+      "buyUnlimited": "Купить Безлимит",
       "manageServers": "Управление серверами",
       "manageServersTitle": "Управление серверами"
     },

src/locales/zh.json

@@ -328,6 +328,7 @@
       "trafficUnavailable": "您的套餐不支持购买流量",
       "unlimited": "无限制",
       "buyTrafficGb": "购买 {{gb}} GB",
+      "buyUnlimited": "购买无限流量",
       "manageServers": "服务器管理",
       "manageServersTitle": "服务器管理"
     },

src/pages/Subscription.tsx

@@ -1588,7 +1588,7 @@ export default function Subscription() {
                         ))}
                       </div>
 
-                      {selectedTrafficPackage &&
+                      {selectedTrafficPackage !== null &&
                         (() => {
                           const selectedPkg = trafficPackages.find(
                             (p) => p.gb === selectedTrafficPackage,
@@ -1625,6 +1625,8 @@ export default function Subscription() {
                                   <span className="flex items-center justify-center gap-2">
                                     <span className="h-4 w-4 animate-spin rounded-full border-2 border-white/30 border-t-white" />
                                   </span>
+                                ) : selectedPkg?.is_unlimited ? (
+                                  t('subscription.additionalOptions.buyUnlimited')
                                 ) : (
                                   t('subscription.additionalOptions.buyTrafficGb', {
                                     gb: selectedTrafficPackage,