Personal, per-browser preference (localStorage), not an admin-wide
Display setting like date/time format — every role can pick their own.
Sets data-bs-theme on <html> to hook into Tabler's built-in dark
variant, applied via an inline script in index.html before React
mounts so there's no light-mode flash on load. Toggle button lives in
the sidebar footer next to Sign out.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>