Add local metrics galleries and timestep charts

[bghira]

Summary

• add step/minutes x-axis support for local scalar charts and prefer validation loss plus step timing metrics
• render validation media as grouped galleries with an image lightbox in the WebUI and offline report
• persist local timestep samples in timestep_distribution.jsonl, expose them through the training metrics API, and render a distribution chart
• auto-refresh running local metrics runs while the Metrics tab is open

Refs #3167.

Validation

• npm test -- --runInBand tests/js/training_metrics_component.test.js
• .venv/bin/python -m unittest tests.test_local_metrics tests.test_training_metrics_service tests.test_iteration_tracker -v -f
• .venv/bin/python -m py_compile simpletuner/helpers/training/trainer.py simpletuner/helpers/training/local_metrics.py simpletuner/simpletuner_sdk/server/services/training_metrics_service.py
• git diff --check

Selenium E2E was attempted with SIMPLETUNER_SELENIUM_TESTS=1 and SELENIUM_SERVER_START_TIMEOUT=120, but the harness failed in setUpClass before the test body because the test server did not become healthy within the timeout.