good-first: aria-busy on error resolve button

[unjica]

Summary

The “Mark as resolved” button shows "…" while pending but does not expose loading state to assistive tech.

File(s)

• apps/dashboard/app/dashboard/errors/ErrorResolveButton.tsx

Tasks

• Add aria-busy="true" while the request is pending
• Add sr-only text such as “Updating status…” during pending state

Acceptance criteria

• Loading state announced in screen readers
• No API/schema changes
• pnpm lint && pnpm test && pnpm build pass

Base branch

Open PRs against develop. See CONTRIBUTING.md.