Fixed intermittent ~60s UI freezes / "Navigation timeout" failures (#4431) caused by heavy third-party imports (matplotlib via the benchmarks package) and synchronous stderr logging blocking the dev server's request pipeline under load. matplotlib/optuna.visualization are now imported lazily (only when benchmark visualizations are generated), and the stderr log sink uses `enqueue=True` so logging never blocks on I/O while holding loguru's handler lock. Note: `enqueue=True` makes stderr logging asynchronous (a background writer thread), so on an abrupt crash the last few buffered log lines may be lost and ordering relative to other sinks can differ slightly.
