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

api/routes/config.py

@@ -29,6 +29,7 @@ class FullConfig(BaseModel):
     proxy: ProxyConfig
     backend: BackendConfig
     allowed_domains: list[DomainEntry]
+    block_message: str = "이 서버는 허용된 도메인에서만 접속 가능합니다."
 
 
 @router.get("/config")