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": {