fix(editor): preserve valid focus and selection state

[DavidBabinec]

What changed

• keep deferred AI composer autofocus from stealing focus already placed inside the assistant panel
• prune invalid selected node ids after both undo and redo restore history
• cover normal autofocus, header-focus retention, undo insertion, and redo deletion regressions

Why

The merged-main browser release gate exposed two user-visible interaction failures: opening the assistant could steal focus back from a panel action, and undoing a selected insertion could leave a stale selection that made the next insert silently no-op.

Verification

• bun test src/__tests__/editor-store/undo-redo.test.ts src/__tests__/panels/agentPanel.test.tsx (43 pass)
• bun run lint
• bunx tsc -b --pretty false
• git diff --check