Fix forced chain auto response index

[RabiMew]

Summary

• track the original chain indices on the client field so forced options can be mapped back to server order
• update the auto-chain handler to return the stored index when responding to forced chains and guard against missing entries
• clear the tracked indices whenever chain selections or the field state are reset

Testing

• cmake -S cmake -B build (fails: missing AutoFiles include and LibEvent package in the build environment)

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https://chatgpt.com/codex/tasks/task_e_68d6e556432c83249111ba30a27901d9