* Unexport GetServerPublicKey, add HealthCheck method
Internalize server key fetching into Login, Register,
GetDeviceAuthorizationFlow, and GetPKCEAuthorizationFlow methods,
removing the need for callers to fetch and pass the key separately.
Replace the exported GetServerPublicKey with a HealthCheck() error
method for connection validation, keeping IsHealthy() bool for
non-blocking background monitoring.
Fix test encryption to use correct key pairs (client public key as
remotePubKey instead of server private key).
* Refactor `doMgmLogin` to return only error, removing unused response
Remove client secret from gRPC auth flow. The secret was originally included to support providers like Google Workspace that don't offer a proper PKCE flow, but this is no longer necessary with the embedded IdP. Deployments using such providers should migrate to the embedded IdP instead.
* 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
