silo/dashboard: auto light/dark + auth recovery button
Theme now honors prefers-color-scheme when the user hasn't explicitly
chosen one, and follows system changes live via MediaQueryList.
On Auth0 init failures (notably stale refresh tokens showing up as
"Unknown or invalid refresh token"), surface a "clear session & retry"
button that nukes the Auth0 localStorage cache and reloads, letting
the user recover without devtools.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>