fix(threads): avoid quadratic overlap check during resume hydration

Summary

Long thread resume could become very slow because overlap detection in resume hydration used nested scans:

• items.some(...) + localItems.some(...) (quadratic in combined list size)

This change replaces that with a Set of local item IDs and O(1) membership checks.

What changed

• In buildResumeHydrationPlan (src/features/threads/utils/threadActionHelpers.ts):
  • build localItemIds = new Set(localItems.map(item => item.id))
  • replace nested overlap check with items.some(item => localItemIds.has(item.id))

Why

For long threads, this removes a hot O(n²) path during resume while keeping behavior identical.

Validation

• npm run -s typecheck
• npm run -s test -- src/features/threads/hooks/useThreadActions.test.tsx src/features/threads/hooks/useThreads.integration.test.tsx src/features/app/hooks/useRemoteThreadRefreshOnFocus.test.tsx src/utils/threadItems.test.ts
• Built and tested aarch64 AppImage locally.