/**
 * Theme identity — portable core. No browser APIs, no React.
 *
 * `UiTheme` is the app-wide theme token. It lives in `src/core/` (not in a
 * store) because pure core logic needs it too: the Vega chart-config mapping
 * (`chartConfigFor`, see docs/architecture/05) keys off the same union, and core
 * must never import from `src/app/`. The store and the document `data-theme`
 * mirror this value; this is its single definition.
 */
export type UiTheme = 'light' | 'dark';