fix: prevent buyer from activating gift pending subscription

When a gift purchase results in pending_activation (recipient has active
subscription), the buyer was shown an Activate button and could replace
the recipient's subscription without consent.

- Add GiftPendingActivationState component showing "Gift sent" message
- Use ?activate=1 URL hint from recipient email to distinguish viewer
- Buyer sees success message, recipient sees activate button
- Add giftPendingActivationDesc translations (ru, en, zh, fa)

src/locales/zh.json

@@ -3548,6 +3548,7 @@
     "credentialsSentToEmail": "登录信息已发送到您的邮箱",
     "giftSentSuccess": "礼物已发送！",
     "giftSentDesc": "收件人将通过邮件收到通知",
+    "giftPendingActivationDesc": "收件人已有活跃订阅。他们将收到激活礼物的链接。",
     "autoLoginFailed": "自动登录失败",
     "autoLoginProcessing": "正在登录...",
     "periodLabels": {