fix: show upgrade prompt when email transcript returns 402 (#13650)

- Show a specific upgrade prompt when free-plan users attempt to send an
email transcript and the API returns a 402 Payment Required error
- Previously, a generic "There was an error, please try again" message
was shown for all failures, including plan restrictions

Fixes
https://linear.app/chatwoot/issue/CW-6538/show-ui-feedback-for-email-transcript-402-plan-restriction

app/javascript/dashboard/i18n/locale/en/conversation.json

@@ -305,6 +305,7 @@
     "CANCEL": "Cancel",
     "SEND_EMAIL_SUCCESS": "The chat transcript was sent successfully",
     "SEND_EMAIL_ERROR": "There was an error, please try again",
+    "SEND_EMAIL_PAYMENT_REQUIRED": "Email transcript is not available on your current plan. Please upgrade to use this feature.",
     "FORM": {
       "SEND_TO_CONTACT": "Send the transcript to the customer",
       "SEND_TO_AGENT": "Send the transcript to the assigned agent",