@abgov/angular-components. No manual
+ document.documentElement.setAttribute in this component —
+ the service owns all DOM and localStorage side effects.
+
+theme = inject(GoabThemeService);
+// template: {{ theme.mode() }}
+// call: theme.toggle() / theme.setMode('dark')
+
+ data-theme attribute → system preference
+ (prefers-color-scheme) → default light.
+ effect() syncs
+ data-theme on <html> and persists to
+ localStorage. Refresh the page — mode persists.
+