fix: resolve all mypy type errors

fpaste

@@ -148,7 +148,8 @@ def request(
 def parse_error(body: bytes, default: str = "request failed") -> str:
     """Parse error message from JSON response body."""
     try:
-        return json.loads(body).get("error", default)
+        result = json.loads(body).get("error", default)
+        return str(result) if result is not None else default
     except (json.JSONDecodeError, UnicodeDecodeError):
         return default
 
@@ -1310,7 +1311,10 @@ def cmd_register(args: argparse.Namespace, config: dict[str, Any]) -> None:
 
         cn = certificate.subject.get_attributes_for_oid(NameOID.COMMON_NAME)
         if cn:
-            print(f"common name: {cn[0].value}", file=sys.stderr)
+            cn_value = cn[0].value
+            if isinstance(cn_value, bytes):
+                cn_value = cn_value.decode("utf-8", errors="replace")
+            print(f"common name: {cn_value}", file=sys.stderr)
 
     print(f"fingerprint: {fingerprint}", file=sys.stderr)
 
@@ -1345,6 +1349,7 @@ def cmd_cert(args: argparse.Namespace, config: dict[str, Any]) -> None:
             die(f"cert file exists: {cert_file} (use --force)")
 
     # Generate private key
+    private_key: rsa.RSAPrivateKey | ec.EllipticCurvePrivateKey
     if args.algorithm == "rsa":
         key_size = args.bits or 4096
         print(f"generating {key_size}-bit RSA key...", file=sys.stderr)