Track JPEG XL validation image support

[bghira]

Background

validation_image_format currently supports png, webp, and jpeg. JPEG XL support should be revisited once mainstream browser display support is available enough for the WebUI and self-contained local metrics reports.

Browser signals moved recently:

• Mozilla posted an Intent to Ship for JPEG XL support on 2026-08-24: https://hacks.mozilla.org/2026/08/intent-to-ship-jpeg-xl/
• Chromium posted an Intent to Ship for JPEG XL decoding support on 2026-08-24: https://groups.google.com/a/chromium.org/g/blink-dev/c/-gDojQbDPRI

Proposed work

• Add jxl to --validation_image_format choices and docs/translations.
• Gate encoding on the existing simpletuner[jxl] extra using pillow-jxl-plugin.
• Save validation images with a .jxl extension when selected.
• Add .jxl media serving and image/jxl content-type handling for the metrics API.
• Add WebUI/offline-report browser tests once target browsers decode image/jxl in normal image elements.

Notes

This should stay a placeholder until browser support and CI/browser test coverage make display behavior reviewable.