fix: restore chat autoscroll on send

[Yezh1i]

Summary

• anchor newly pending user messages so sending from a scrolled-up conversation returns to the active turn
• preserve the existing parent-user anchor behavior for assistant-only retries and continuations
• add focused regression coverage for live-message anchor selection

Root cause

Pending user messages are marked isPending, but the live-anchor lookup started one item before the first live message. As a result, the newly appended user item was skipped and the message scroller could not find a new scrollAnchor to jump to when the viewport was away from the bottom.

Testing

• pnpm --filter @deeix/web test
• pnpm --filter @deeix/web typecheck
• pnpm --filter @deeix/web lint
• pnpm --filter @deeix/web build

[Yezh1i]

Superseded by #523, which contains the verified final implementation as a clean single commit.