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/store/modules/conversations/actions.js

@@ -457,11 +457,7 @@ const actions = {
   },
 
   sendEmailTranscript: async (_, { conversationId, email }) => {
-    try {
-      await ConversationApi.sendEmailTranscript({ conversationId, email });
-    } catch (error) {
-      throw new Error(error);
-    }
+    await ConversationApi.sendEmailTranscript({ conversationId, email });
   },
 
   updateCustomAttributes: async (