💡 [REQUEST] - In-thread message search on Inbox conversation view

## Summary
Add the ability to search within the currently open conversation on the Inbox page: type a query, jump between matches, and highlight results in the message list.

## Motivation
Support agents need to quickly find prior context (e.g., an account number, ticket ID, or a promise made earlier) without scanning hundreds of messages. In-thread search reduces handling time and errors.
