feat: use gemini tts for livekit voice

.env.example

@@ -11,6 +11,7 @@ GEMINI_API_KEY=
 # canonical deployment secret name used by DigitalOcean and docs.
 GEMINI_VISION_MODEL=gemini-2.0-flash
 GEMINI_LIVE_MODEL=gemini-3.1-flash-tts-preview
+GEMINI_TTS_VOICE=Charon
 GEMINI_EMBEDDING_MODEL=gemini-embedding-001
 
 # --- GitHub (repo state + sync PR artifacts) ---
@@ -32,6 +33,8 @@ NUDGE_COOLDOWN_MS=180000
 # --- Frontend (Vite — must be VITE_ prefixed to reach the client) ---
 VITE_LIVEKIT_URL=wss://your-project.livekit.cloud
 VITE_BACKEND_URL=http://localhost:8787
+# Keep off by default so users hear Gemini audio delivered through LiveKit.
+VITE_ENABLE_BROWSER_TTS_FALLBACK=false
 
 # --- Deployment verification ---
 # Optional override when the deployed SPA and API use different origins.