fix(ui): remove blocks holder listeners and detach it on destroy

[SAY-5]

Closes #170

BlocksUI attaches beforeinput and keydown listeners to the blocks holder in #prepareBlocksHolder() but destroy() never removes them, and the holder itself is never detached. If a holder is reused or a new instance is created, the old listeners keep firing alongside the new ones (duplicate BeforeInputUIEvent dispatches and stale undo/redo handling).

This wires both listeners to a single AbortController and calls .abort() in destroy(), which is the pattern the issue suggested. destroy() now also detaches #blocksHolder, and EditorjsUI.destroy() (previously a no-op) removes its #editorWrapper so the whole subtree comes off the page.

The ui package has no test runner wired up yet (no Blocks.spec.ts and no test script), so this doesn't add a spec. Happy to follow up with one if you want the test harness set up here.