feat: add auto theme with system dark/light preference detection}
- Add "Auto" theme option that follows OS prefers-color-scheme setting
- Implement flash-free server-side rendering for auto theme via inline
scripts in layout.tpl.twig to set correct stylesheet before paint
- Listen for system preference changes and switch theme dynamically
- Add is_auto_theme Twig global in Application.php for SSR support
- Add nav.theme_auto translation key across all 27 language files
- Change navbar background from bg-primary to bg-dark
- Fix collapsed navbar dropdowns to expand inline with static position
instead of separate scroll containers
- Update CSS postfix hash and theme asset paths
- Add v2026.4.319 changelog entry
- Bump version to v2026.4.320
- Remove AGENTS.md