computeHash method
Computes the SHA-256 hash of a file at filePath.
Used for integrity verification: the sender computes the hash when creating the message, and the receiver verifies after the FILE transfer completes.
Implementation
Future<String> computeHash(String filePath) async {
final file = File(filePath);
final bytes = await file.readAsBytes();
return _computeHashFromBytes(bytes);
}