development-nightly-2026-10-15

feat(layout): add a "Force layout mode" configuration, closes #4762

Administrators can now mandate a single color mode (light, dark or
high-contrast) for every visitor via the new layout.forceLayoutMode
setting. A forced mode is rendered server-side into data-bs-theme; the
inline theme script is omitted and the ThemeSwitcher chunk is no longer
loaded on public pages, so neither localStorage nor the OS preference can
override it.

The switcher is now an on-demand chunk (dynamic import, no barrel export)
instead of a side effect shipped with every public page. The inline script
also honours the default layout mode and the visitor-choice flag.