Track agent connection status in the configuration UI, show a
restart action for connected custom agents, and only render the
External Agents menu section when entries exist.

<img width="505" height="218" alt="image"
src="https://github.com/user-attachments/assets/117edd94-dd06-4b1f-a530-308b7219404b"
/>


Release Notes:

- acp: Allow for restarting agent servers from the Agent Settings panel.

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Co-authored-by: MrSubidubi <dev@bahn.sh>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>