feat(FR-1472): implement GraphQL viewer query for current user information (#4310)

Resolves #4279 ([FR-1472](https://lablup.atlassian.net/browse/FR-1472))

## Summary
Implements GraphQL `viewer` query functionality to handle current user information and determine user roles.

## Changes
- Added `Viewer` type to GraphQL schema with user information fields
- Created `useViewer` hook for accessing current user data via GraphQL
- Enhanced BAI Client Provider with viewer-related hooks and type definitions
- Integrated viewer functionality into React hooks system

## Context
This implementation is based on the backend changes from lablup/backend.ai#5728, which introduces the Viewer functionality for retrieving authenticated user information.

## Test Plan
- [ ] Verify `useViewer` hook returns current user information correctly
- [ ] Confirm user role is properly determined from viewer query
- [ ] Test integration with existing authentication system
- [ ] Ensure GraphQL schema updates are compatible with backend

[FR-1472]: https://lablup.atlassian.net/browse/FR-1472?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: yomybaby