feat: send Login Disconnect with custom message on blocked join

Previously a blocked join just dropped the socket, so the MC client
showed 'Internal Exception: SocketException: Connection reset'.

Now when next_state=2 (login), the proxy sends a proper Login
Disconnect (0x00) packet containing a JSON chat component, and the
client displays the message on its disconnect screen.

- block_message added to config (default Korean message); editable
  in Settings UI as a textarea
- build_login_disconnect() encodes (varint length)+(0x00)+(JSON str)
- Status/ping (next_state=1) still silently dropped so the proxy
  presence is not announced to scanners
- Backward-compat: load_config() backfills block_message on old files

proxy/config.py

@@ -19,6 +19,7 @@ DEFAULT_CONFIG = {
     "allowed_domains": [
         {"domain": "mc.tkrmagid.kr", "enabled": True, "note": "메인 도메인"}
     ],
+    "block_message": "이 서버는 허용된 도메인에서만 접속 가능합니다.",
 }
 
 _lock = threading.Lock()