fix: animate chat scroll after send

[Yezh1i]

Summary

• follow a newly submitted user turn even when the user sends from an older scroll position
• animate the message viewport to the latest turn over 700 ms instead of jumping instantly
• keep the previous user message as the streaming resize anchor
• wait for pending-message layout observers before starting the animation
• add regression coverage for trigger selection, anchor selection, scheduling, timing, and cancellation

This is the clean, single-commit replacement for #522.

Why

The message scroller's built-in scrollToEnd switches into automatic follow mode. Its resize handling can then overwrite native smooth scrolling with an immediate scrollTop update. The submitted-message path now uses a controlled requestAnimationFrame animation on the viewport while preserving the scroller's existing streaming anchor behavior.

Verification

• pnpm --filter @deeix/web test — 12 passed
• pnpm --filter @deeix/web typecheck — passed
• pnpm --filter @deeix/web lint — passed
• pnpm --filter @deeix/web build — passed
• git diff --check — passed
• manually verified in the local Docker SQLite deployment