♻️(frontend) sync publishing sources with Django backend settings

Replace hardcoded default publishing source constants with values from
Django backend settings to prevent desynchronization between frontend
and backend configurations.

src/backend/core/api/__init__.py

@@ -55,6 +55,7 @@ def get_frontend_configuration(request):
             "url": settings.LIVEKIT_CONFIGURATION["url"],
             "force_wss_protocol": settings.LIVEKIT_FORCE_WSS_PROTOCOL,
             "enable_firefox_proxy_workaround": settings.LIVEKIT_ENABLE_FIREFOX_PROXY_WORKAROUND,
+            "default_sources": settings.LIVEKIT_DEFAULT_SOURCES,
         },
     }
     frontend_configuration.update(settings.FRONTEND_CONFIGURATION)