fix(i18n): rename subscription.servers object to subscription.serverManagement

Resolves conflict where subscription.servers existed as both a nested
object (for server management UI keys) and a pluralization base key.
Renamed the nested object to serverManagement in all 4 locales and
updated all t() references in Subscription.tsx.

src/locales/zh.json

@@ -242,7 +242,7 @@
       "manageServers": "服务器管理",
       "manageServersTitle": "服务器管理"
     },
-    "servers": {
+    "serverManagement": {
       "statusLegend": "✅ — 已连接 • ➕ — 将添加（付费）• ➖ — 将断开",
       "discountBanner": "您的服务器折扣：-{{percent}}%",
       "forDays": "（{{days}} 天）",