* Consolidate authentication logic
- Moving auth functions from client/internal to client/internal/auth package
- Creating unified auth.Auth client with NewAuth() constructor
- Replacing direct auth function calls with auth client methods
- Refactoring device flow and PKCE flow implementations
- Updating iOS/Android/server code to use new auth client API
* Refactor PKCE auth and login methods
- Remove unnecessary internal package reference in PKCE flow test
- Adjust context assignment placement in iOS and Android login methods
* Add OAuth select_account prompt support to PKCE flow
Extends LoginFlag enum with select_account options to enable
multi-account selection during authentication. This allows users
to choose which account to use when multiple accounts have active
sessions with the identity provider.
The new flags are backward compatible - existing LoginFlag values
(0=prompt login, 1=max_age=0) retain their original behavior.
