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/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);
 }