fix: don't force auto-scroll when user scrolled up during streaming

[ebarroca]

Summary

• When streaming is active, the conversation auto-scrolled to the bottom unconditionally, yanking the user back down even if they had scrolled up to read earlier content
• Now tracks user scroll position and suppresses auto-scroll when the user has scrolled away from the bottom
• Auto-scroll resumes naturally when the user scrolls back to the bottom

Test plan

• Start an agent conversation that produces streaming output
• While streaming, scroll up to read earlier messages
• Verify you stay where you are and are NOT pulled back to the bottom
• Scroll back to the bottom and verify auto-scroll resumes

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