[FEATURE]: TUI sidebar overlay behaviour ought to be configurable. #6086

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Often I keep opencode in the leftmost third of my screen adjacent my text editor, so this new behaviour of turning the sidebar into an overlay has been fairly disruptive: I don't want the session content being dimmed or its text being hidden beneath the sidebar, I just want a narrower view of the section content in the space not occupied by the sidebar.

This behaviour ought to be configurable so that users who don't care for this new behaviour can disable this new sidebar overlay behaviour and get back the old behaviour where the sidebar could happily coexist with the session content within the confines of a narrow window.