Initial commit: overlay + sender pages for StreamLabs browser source

Express + Socket.IO backend with Google login on the sender page,
a transparent overlay page for OBS/StreamLabs Browser Source, and
TTS voice-over via the edge-tts Python CLI (same engine as
t2sTelegramBot). Dockerized for deployment.

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

public/overlay.js

@@ -0,0 +1,72 @@
+const box = document.getElementById('box');
+const fromEl = document.getElementById('from');
+const textEl = document.getElementById('text');
+const socket = io();
+
+let hideTimer = null;
+let audioCtx = null;
+
+function getAudioCtx() {
+  audioCtx = audioCtx || new (window.AudioContext || window.webkitAudioContext)();
+  return audioCtx;
+}
+
+// Browsers suspend AudioContext until a user gesture happens on the page.
+// OBS's Browser Source doesn't enforce this, but for testing in a normal
+// tab, unlock it on the first click/keypress anywhere on the page.
+['click', 'keydown'].forEach((eventName) => {
+  document.addEventListener(eventName, () => {
+    const ctx = getAudioCtx();
+    if (ctx.state === 'suspended') ctx.resume();
+  }, { once: true });
+});
+
+function playNotificationSound() {
+  try {
+    const ctx = getAudioCtx();
+    const osc = ctx.createOscillator();
+    const gain = ctx.createGain();
+    osc.type = 'sine';
+    osc.frequency.setValueAtTime(880, ctx.currentTime);
+    osc.frequency.setValueAtTime(1320, ctx.currentTime + 0.1);
+    gain.gain.setValueAtTime(0.2, ctx.currentTime);
+    gain.gain.exponentialRampToValueAtTime(0.001, ctx.currentTime + 0.35);
+    osc.connect(gain);
+    gain.connect(ctx.destination);
+    osc.start();
+    osc.stop(ctx.currentTime + 0.35);
+  } catch (err) {
+    console.error('Could not play notification sound:', err);
+  }
+}
+
+function showMessage(text, from) {
+  clearTimeout(hideTimer);
+
+  fromEl.textContent = from || '';
+  fromEl.style.display = from ? 'block' : 'none';
+  textEl.textContent = text;
+  // force reflow so the transition re-triggers even if a message is already visible
+  void box.offsetWidth;
+  box.classList.add('visible');
+
+  playNotificationSound();
+
+  const durationMs = 10000;
+  hideTimer = setTimeout(() => {
+    box.classList.remove('visible');
+  }, durationMs);
+}
+
+function playSpeech(base64Audio, mimeType) {
+  const audio = new Audio(`data:${mimeType};base64,${base64Audio}`);
+  audio.play().catch((err) => console.error('Could not play TTS audio:', err));
+}
+
+socket.on('display_message', (payload) => {
+  showMessage(payload.text, payload.from);
+});
+
+socket.on('display_audio', (payload) => {
+  playSpeech(payload.audio, payload.mimeType);
+});