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

frontend/src/pages/Settings.jsx

@@ -51,6 +51,23 @@ export default function Settings() {
           프록시 활성화
         </label>
       </section>
+      <section className="card">
+        <h2>차단 메시지</h2>
+        <label>
+          허용되지 않은 도메인으로 접속 시도 시 마인크래프트 클라이언트에 보여줄 메시지
+          <textarea
+            rows={2}
+            value={cfg.block_message ?? ''}
+            onChange={(e) =>
+              setCfg({ ...cfg, block_message: e.target.value })
+            }
+            placeholder="이 서버는 허용된 도메인에서만 접속 가능합니다."
+          />
+          <span className="muted">
+            로그인(join) 시도에만 적용됩니다. 서버 리스트 핑은 그냥 응답을 차단합니다.
+          </span>
+        </label>
+      </section>
       <section className="card">
         <h2>백엔드 (실제 MC 서버)</h2>
         <label>

frontend/src/styles.css

@@ -159,7 +159,8 @@ button.mt { margin-top: 12px; }
 input:disabled { opacity: 0.7; cursor: not-allowed; }
 
 input,
-select {
+select,
+textarea {
   background: var(--bg);
   color: var(--text);
   border: 1px solid var(--border);
@@ -168,8 +169,15 @@ select {
   font-size: 14px;
   font-family: inherit;
 }
+textarea {
+  width: 100%;
+  max-width: 520px;
+  resize: vertical;
+  margin-top: 4px;
+}
 input:focus,
-select:focus {
+select:focus,
+textarea:focus {
   outline: none;
   border-color: var(--accent);
 }