`PublicKey.String()`, `URLEncoded()`, `Bytes()`, and `Equal()` all panic when called on a `nil` receiver. A peer can exploit this by sending a Manifest with a missing `network_id` or `signed_by`, causing a nil-pointer panic in the stream-handler goroutine that crashes the daemon.

Add nil-receiver guards to `PublicKey` methods and an early nil-check
in `SignatureAuth.Authorize()` so malformed signatures are rejected
with an error rather than crashing the process.

@@ -51,6 +51,10 @@ func NewSignatureAuth(cfg *Config) *SignatureAuth {
 }
 
 func (sf *SignatureAuth) Authorize(sig *model.Signature) error {
+	if sig.NetworkID == nil || sig.SignedBy == nil {