feat(confidence-engine): add dark mode

lib/theme-preference.js

@@ -0,0 +1,22 @@
+export const THEME_STORAGE_KEY = "confidence-engine-theme";
+
+export function resolveTheme({ savedTheme, systemPrefersDark = false } = {}) {
+  if (savedTheme === "light" || savedTheme === "dark") return savedTheme;
+  return systemPrefersDark ? "dark" : "light";
+}
+
+export function toggleTheme(theme) {
+  return theme === "dark" ? "light" : "dark";
+}
+
+export function readThemePreference(storage) {
+  try {
+    return storage?.getItem(THEME_STORAGE_KEY) ?? null;
+  } catch {
+    return null;
+  }
+}
+
+export function saveThemePreference(theme, storage) {
+  storage?.setItem(THEME_STORAGE_KEY, theme);
+}