/**
 * Extracts a user-facing error message from any thrown value.
 * For AppError with validation details, flattens the field-level messages so the
 * UI shows the actual cause (e.g. "Unsupported content type 'image/bmp'") rather than
 * a generic "Validation Failed" title.
 */
export declare function extractErrorMessage(error: unknown): string;
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