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

cmd/canvasarchiver/main.go

@@ -17,19 +17,12 @@ func main() {
 	videosOnly := flag.Bool("vo", false, "Videos only mode - download only Panopto videos to a single directory")
 	me := flag.Bool("me", false, "Download all enrolled courses")
 	moduleNumbers := flag.Bool("n", false, "Prefix modules with order numbers [1], [2], etc.")
-	apiToken := flag.Bool("api", false, "Use a manually generated Canvas API token instead of OAuth")
 	flag.Parse()
 
 	httpClient := &http.Client{}
 
 	authenticator := auth.NewAuthenticator(httpClient)
-	var accessToken string
-	var err error
-	if *apiToken {
-		accessToken, err = authenticator.GetAPIToken()
-	} else {
-		accessToken, err = authenticator.GetAccessToken()
-	}
+	accessToken, err := authenticator.GetToken()
 	if err != nil {
 		fmt.Printf("Authentication failed: %v\n", err)
 		return