fix: keep terminal viewport at bottom on enter

[qfxiongbinbin]

Problem

Pressing Enter in the terminal can jump the viewport to the top instead of keeping the prompt anchored near the bottom.

What changed

• Added a small helper to detect whether the terminal viewport is already near the bottom
• When replaying buffered output, resizing the terminal, or writing a new chunk, keep the viewport pinned to the bottom only if the user was already near the bottom
• Avoid forcing scroll when the user has intentionally scrolled up into history

Expected result

• Pressing Enter in a normal terminal session no longer jumps the viewport to the top
• Users can still scroll up and inspect history without the app fighting them