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/canvas/client.go

@@ -16,6 +16,13 @@ import (
 	"git.directme.in/Joren/CanvasArchiver/internal/utils"
 )
 
+func checkAPIResponse(resp *http.Response) error {
+	if resp.StatusCode < 300 {
+		return nil
+	}
+	return fmt.Errorf("API error: %d", resp.StatusCode)
+}
+
 type Client struct {
 	HTTPClient      *http.Client
 	AccessToken     string
@@ -48,6 +55,10 @@ func (c *Client) GetCourseInfo() error {
 	}
 	defer resp.Body.Close()
 
+	if err := checkAPIResponse(resp); err != nil {
+		return err
+	}
+
 	var course models.Course
 	json.NewDecoder(resp.Body).Decode(&course)
 
@@ -64,6 +75,10 @@ func (c *Client) GetEnrolledCourses() ([]models.Course, error) {
 	}
 	defer resp.Body.Close()
 
+	if err := checkAPIResponse(resp); err != nil {
+		return nil, err
+	}
+
 	var courses []models.Course
 	json.NewDecoder(resp.Body).Decode(&courses)
 	return courses, nil