Allow a fixed loopback port in blogger-auth

Supports web-application OAuth clients with a pre-registered
http://127.0.0.1:<port> redirect URI; desktop clients keep the random
port default.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

us2kr/cli.py

@@ -95,6 +95,13 @@ def build_parser() -> argparse.ArgumentParser:
         "--client-secret", default=None, help="OAuth client secret (default: BLOGGER_CLIENT_SECRET)"
     )
     auth.add_argument("--timeout", type=int, default=300, help="seconds to wait for the browser consent")
+    auth.add_argument(
+        "--port",
+        type=int,
+        default=0,
+        help="fixed loopback port (0 = random). Use with a *web application* OAuth client whose "
+        "authorized redirect URI is registered as http://127.0.0.1:<port>",
+    )
     return parser
 
 
@@ -191,7 +198,7 @@ def cmd_blogger_auth(args: argparse.Namespace, settings: Settings) -> int:
         def log_message(self, *_args: object) -> None:  # keep the terminal clean
             pass
 
-    server = HTTPServer(("127.0.0.1", 0), Handler)
+    server = HTTPServer(("127.0.0.1", args.port), Handler)
     port = server.server_address[1]
     redirect_uri = f"http://127.0.0.1:{port}"
     threading.Thread(target=server.serve_forever, daemon=True).start()