Add optional AI reply via DeepSeek

Viewers can check a box on the send page to get a short AI-generated
reply to their message, voiced with a distinct TTS voice and queued
on the overlay after the original message. The system prompt lives
in ai-prompt.txt (editable without touching code) instead of being
hardcoded. TTS for both viewer messages and AI replies now announces
who it's from ("Сообщение от X." / "Ответ от ИИ.") before the text.
No-ops entirely if DEEPSEEK_API_KEY isn't set.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/send.js

@@ -5,6 +5,7 @@ const userName = document.getElementById('user-name');
 const logoutBtn = document.getElementById('logout');
 const displayNameInput = document.getElementById('display-name');
 const messageInput = document.getElementById('message');
+const aiReplyCheckbox = document.getElementById('ai-reply');
 const sendBtn = document.getElementById('send');
 const statusEl = document.getElementById('status');
 
@@ -74,7 +75,8 @@ sendBtn.addEventListener('click', () => {
   const text = messageInput.value.trim();
   if (!text) return;
   const name = displayNameInput.value.trim();
-  socket.emit('send_message', { token, text, name });
+  const aiReply = aiReplyCheckbox.checked;
+  socket.emit('send_message', { token, text, name, aiReply });
   messageInput.value = '';
   statusEl.textContent = 'Отправлено';
   setTimeout(() => { statusEl.textContent = ''; }, 1500);