Fix email thread polling causing duplicate messages

[Neverdecel]

Summary

• Fix duplicate messages appearing in email thread view during polling
• Replace complex DOM manipulation with simple page reload approach
• Track messages by ID instead of array index for accurate new message detection

Problem

When sending a message in the email thread:

1. The sent message didn't appear in the conversation
2. The agent's response appeared as a duplicate of an existing message
3. After manual page refresh, everything displayed correctly

Root Cause

The JavaScript polling logic used innerHTML manipulation and array index-based comparison that caused messages to be appended incorrectly, leading to duplicates. The messages are displayed newest-first, but the old logic assumed oldest-first ordering.

Solution

Changed the polling behavior to:

1. Track existing messages by their unique IDs using a Set
2. Compare fetched messages against existing IDs to detect new ones
3. Simply reload the page when new messages are detected

This is simpler and more reliable than trying to surgically update the DOM.

Testing

• Verified sent messages appear correctly at the top of the thread
• Verified agent responses appear correctly without duplicates
• Verified message count updates correctly
• Tested with multiple message exchanges

DevOps Notes

No infrastructure changes required. This is a frontend-only fix in a Jinja2 template.