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()

proxy/main.py

@@ -10,6 +10,7 @@ asyncio 기반 TCP 프록시. 동작 순서:
 from __future__ import annotations
 
 import asyncio
+import json
 import logging
 import os
 import sqlite3
@@ -19,6 +20,39 @@ from pathlib import Path
 import config as cfg_mod
 from handshake import HandshakeError, parse_handshake, read_handshake_bytes
 
+DEFAULT_BLOCK_MESSAGE = "이 서버는 허용된 도메인에서만 접속 가능합니다."
+
+
+def _encode_varint(n: int) -> bytes:
+    out = bytearray()
+    while True:
+        b = n & 0x7F
+        n >>= 7
+        if n:
+            out.append(b | 0x80)
+        else:
+            out.append(b)
+            break
+    return bytes(out)
+
+
+def build_login_disconnect(message: str) -> bytes:
+    """Login Disconnect (clientbound, login state, packet id 0x00).
+
+    Body: JSON chat component as length-prefixed UTF-8 string.
+    클라이언트는 이 패킷을 받으면 "서버에서 연결을 거부했습니다" 화면 대신
+    여기 담긴 chat 컴포넌트를 그대로 보여준다.
+    """
+    chat_json = json.dumps(
+        {"text": message, "color": "red"}, ensure_ascii=False
+    ).encode("utf-8")
+    body = (
+        _encode_varint(0x00)
+        + _encode_varint(len(chat_json))
+        + chat_json
+    )
+    return _encode_varint(len(body)) + body
+
 LOG_DB = Path(os.environ.get("MC_LOG_DB", "/data/logs.db"))
 RESTART_SIGNAL = Path(os.environ.get("MC_RESTART_SIGNAL", "/data/restart.signal"))
 
@@ -184,6 +218,15 @@ async def handle_client(
         log.info(
             "BLOCK %s domain=%r next_state=%d", client_ip, domain, hs.next_state
         )
+        # next_state=2 (login) 인 경우 Login Disconnect 패킷으로 메시지 전달,
+        # next_state=1 (status/ping) 은 그냥 끊는다 (프록시 존재 자체를 굳이 노출 X).
+        if hs.next_state == 2:
+            try:
+                msg = state.cfg.get("block_message") or DEFAULT_BLOCK_MESSAGE
+                client_writer.write(build_login_disconnect(msg))
+                await client_writer.drain()
+            except (OSError, ConnectionResetError, BrokenPipeError):
+                pass
         client_writer.close()
         return