[FEATURE]: Preserve conversation scroll position when returning to a session #28120

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When a user scrolls up in a session, switches to another session or route, then returns, OpenCode should restore the conversation view to the scroll position where the user left it.

Current behavior resets the view, usually to the bottom or another default position. This makes it hard to compare context across sessions or continue reading older output.

Expected behavior:

1. Open a session with enough history to scroll.
2. Scroll to an older point in the conversation.
3. Switch to another session.
4. Return to the original session.
5. The conversation view is restored near the same message/offset.

Notes:

• This should be per session.
• If the session has new messages and the user was already at the bottom, staying pinned to bottom is fine.
• If the user had manually scrolled up, returning should preserve that reading position.
• Related scroll-state issues: Session switching gets progressively slower as message history grows #17775, Setting to prevent TUI scrolling when new message are streamed-in #7648, Don't Automatically Scroll Chat Window #7659, [Bug] TUI scroll position resets to session start after approving commands that exceed terminal height #6895.