refactor(ui): thin Team setup dialog shell (#1554)

## Description
Replaces the legacy Team setup god component with a 232-line coordination shell and a presentational dialog view. The opener no longer uses mutable module globals; concurrent item work renders correctly; scoped errors have valid accessible references and unique retry controls.

Part 3 of codemem-752i.4.

## Type of Change
- [ ] 🚀 Feature (new functionality)
- [x] 🐛 Bug fix (fixes an issue)
- [ ] 📚 Documentation (docs-only change)
- [x] 🔧 Maintenance (refactor, chore, CI, etc.)
- [ ] 🧪 Testing (test-only changes)

## Testing
- [x] 79 focused reducer, effect, focus, dialog, and app tests
- [x] `pnpm run tsc`
- [x] `pnpm run lint`
- [x] `pnpm --filter @codemem/ui build`

## Checklist
- [x] Code follows project style
- [x] CodeReviewer and complexity review completed
- [x] No new warnings introduced