Add `no_std` support for keetanetwork-account to enable ledger device compatibility

[mamonet]

Add no_std support to keetanetwork-account to enable use in embedded environments like Ledger hardware wallets. The ledger device app needs to display human-readable account addresses (e.g., keeta_...). Currently, keetanetwork-account requires std, making it unusable on the ledger platform.

• format_public_key_string() - encode pubkey to keeta_... address
• parse_public_key() - decode keeta_... address to pubkey bytes
• Key type constants (ECDSA, ED25519, TOKEN, NETWORK, etc.)