🐛(frontend) add LiveKit connection warmup for Firefox+proxy fixes

Implement HTTPS prefetch before joining rooms to resolve WebSocket
handshake failures where Firefox+proxy returns HTTP 200 instead of 101.

Reproduced locally with Squid container. No proxy configuration fixes
found - HTTPS warmup is only working workaround. Issue doesn't occur
when signaling server shares webapp domain, making warmup unnecessary.

Use HEAD request to minimize bandwidth.

src/backend/core/api/__init__.py

@@ -50,6 +50,7 @@ def get_frontend_configuration(request):
             else None,
             "default_country": settings.ROOM_TELEPHONY_DEFAULT_COUNTRY,
         },
+        "livekit": {"url": settings.LIVEKIT_CONFIGURATION["url"]},
     }
     frontend_configuration.update(settings.FRONTEND_CONFIGURATION)
     return Response(frontend_configuration)