Theme controller
DarkLightSystem
pref: dark · effective: dark. Controller persists to localStorage, resolves `system` via matchMedia, and syncs across windows via the storage event — all started in an effect (SSR-safe).
pref: dark · effective: dark. Controller persists to localStorage, resolves `system` via matchMedia, and syncs across windows via the storage event — all started in an effect (SSR-safe).