reworked api interfaces

pkg/garth/auth/sso/doc.go

@@ -0,0 +1,5 @@
+// Package sso implements the Garmin SSO login flow. It orchestrates CSRF,
+// ticket exchange, MFA placeholders, and token retrieval, delegating OAuth
+// details to internal/auth/oauth. The internal client consumes this package.
+// Note: This is an internal package and not intended for direct external use.
+package auth