Enhance NEAR Smart Contract Security with Access Controls

NEAR accounts include user-friendly named (2-64 chars) and cryptographic implicit types. Permissions split between Full Access and restrictive Function Call Keys. Security mandates using predecessor_account_id() over signer_account_id() for access control. The #[private] macro secures callbacks but can be bypassed if account keys call contract functions directly. Understanding this hierarchy improves NEAR contract robustness and security.