feat: interactive auth method selection with API token validation

- Add GetToken() as unified auth entry point with method selection
- Prompt [1] OAuth / [2] API token on first launch, save choice
- Validate API tokens against /api/v1/users/self before saving
- Check saved API token validity on reuse; fallback to re-prompt on failure
- Add HTTP status check to GetCourseInfo and GetEnrolledCourses
- Remove --api flag, auth method is now persistent and interactive

internal/models/models.go

@@ -1,6 +1,7 @@
 package models
 
 type Credentials struct {
+	AuthMethod   string `json:"auth_method,omitempty"`
 	RefreshToken string `json:"refresh_token,omitempty"`
 	APIToken     string `json:"api_token,omitempty"`
 }