`useLayoutEffect` is deprecated in OWL3. This replaces it with `useEffect`
and moves the vertical-positioning logic into a `threadTops` getter.

Two `useLayoutEffect` calls existed: one ran a batched vertical-dimension
computation on `activeThreadId` change, the other a debounced horizontal
computation on `editorThreads` change.

- Horizontal: kept as a `useEffect`. Where the old hook ran on every patch,
  the OWL3 effect only reacts to the values it reads, so it reruns just when
  threads are added or removed.
- Vertical: the positions can simply be derived while rendering, so the logic
  moved into a `threadTops` getter used by the template. This lets the render
  react to the relevant state on its own and drops the post-patch hook.

The refactored code has test coverage — these failed when the effect was
commented out and now pass:
- TestESLint.test_eslint (`activeThreadId` defined but never used)
- TestKnowledgeArticleTours.test_knowledge_article_comments (comments UI broken)
runbot build with the effect commented out: https://runbot.odoo.com/runbot/batch/2594694/build/114736755