Fix: Restore main chat scroll position after closing thread view

[DammyCodes-all]

Brief Title

Prevent jump to old messages when returning from thread to main chat

Acceptance Criteria fulfillment

• Task 1: Preserve main chat scroll position before entering thread view
• Task 2: Restore preserved scroll position after exiting thread view
• Task 3: Prevent accidental older-message pagination during thread->chat restore

Fixes #1177

PR Test Details

• Open EmbeddedChat in a channel with enough history.
• Scroll to a non-latest position in main chat.
• Open a thread from a message.
• Click the top-left back arrow.
• Verify main chat returns to the previous position instead of jumping to old/top history.
• Verify older-message fetch is not triggered immediately during restore.

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-1178 after approval. Contributors are requested to replace 1178 with the actual PR number.

[CLAassistant]

All committers have signed the CLA.