Show search-key & prefix in search boxes if search is enabled (GSoC 2026)

### Bug description <!-- If discussed in #zulip-terminal or another channel on chat.zulip.org, paste link below: --> ### How is the bug triggered? How can you reproduce the bug? 1. ### What did you expect to happen? ### How are you running the application? Please include as many of the following as possible: - **Zulip-terminal version:** eg. a specific version (0.7.0), or if running from `main` also ideally the git ref - **Zulip server version(s):** eg. Zulip Cloud, the version you are running self-hosted, or the Zulip Community server (chat.zulip.org) - **Operating system (and version):** eg. Debian Linux, Ubuntu Linux, macOS, WSL in Windows, Docker - **Python version (and implementation):** eg. 3.8, 3.9, 3.10, ... (implementation is likely to be eg. CPython, or PyPy) If possible, please provide details from the `About` menu: (hotkey: Meta + ?) (this can provide some of the details above)