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/en.json

@@ -3998,6 +3998,7 @@
     "credentialsSentToEmail": "Login credentials sent to your email",
     "giftSentSuccess": "Gift sent!",
     "giftSentDesc": "Recipient will be notified by email",
+    "giftPendingActivationDesc": "The recipient already has an active subscription. They will receive a link to activate the gift.",
     "autoLoginFailed": "Auto-login failed",
     "autoLoginProcessing": "Signing in...",
     "periodLabels": {

src/locales/fa.json

@@ -3549,6 +3549,7 @@
     "credentialsSentToEmail": "اطلاعات ورود به ایمیل شما ارسال شد",
     "giftSentSuccess": "هدیه ارسال شد!",
     "giftSentDesc": "گیرنده از طریق ایمیل مطلع خواهد شد",
+    "giftPendingActivationDesc": "گیرنده در حال حاضر اشتراک فعال دارد. لینک فعال‌سازی هدیه برای او ارسال خواهد شد.",
     "autoLoginFailed": "ورود خودکار ناموفق بود",
     "autoLoginProcessing": "در حال ورود...",
     "periodLabels": {

src/locales/ru.json

@@ -4557,6 +4557,7 @@
     "credentialsSentToEmail": "Данные для входа отправлены на email",
     "giftSentSuccess": "Подарок отправлен!",
     "giftSentDesc": "Получатель получит уведомление на email",
+    "giftPendingActivationDesc": "У получателя уже есть активная подписка. Ему будет отправлена ссылка для активации подарка.",
     "autoLoginFailed": "Не удалось выполнить автоматический вход",
     "autoLoginProcessing": "Выполняется вход...",
     "periodLabels": {

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": {