export interface PkcePair {
    verifier: string;
    challenge: string;
    method: 'S256';
}
/**
 * Generate a PKCE verifier + challenge pair (S256).
 * Verifier: 32 random bytes, base64url-encoded.
 * Challenge: SHA-256(verifier), base64url-encoded.
 */
export declare function generatePkce(): PkcePair;