feat(auth): add CodeBuddy-CN browser OAuth authentication support

internal/auth/codebuddy/errors.go

@@ -0,0 +1,25 @@
+package codebuddy
+
+import "errors"
+
+var (
+	ErrPollingTimeout   = errors.New("codebuddy: polling timeout, user did not authorize in time")
+	ErrAccessDenied     = errors.New("codebuddy: access denied by user")
+	ErrTokenFetchFailed = errors.New("codebuddy: failed to fetch token from server")
+	ErrJWTDecodeFailed  = errors.New("codebuddy: failed to decode JWT token")
+)
+
+func GetUserFriendlyMessage(err error) string {
+	switch {
+	case errors.Is(err, ErrPollingTimeout):
+		return "Authentication timed out. Please try again."
+	case errors.Is(err, ErrAccessDenied):
+		return "Access denied. Please try again and approve the login request."
+	case errors.Is(err, ErrJWTDecodeFailed):
+		return "Failed to decode token. Please try logging in again."
+	case errors.Is(err, ErrTokenFetchFailed):
+		return "Failed to fetch token from server. Please try again."
+	default:
+		return "Authentication failed: " + err.Error()
+	}
+}