feat: support query theme bootstrap for embeds

lib/theme-sync.ts

@@ -1,6 +1,25 @@
 import type { Theme } from "./theme";
 
 export type ResolvedTheme = "light" | "dark";
+export type ThemeQueryMode = "light" | "dark" | "system";
+
+export function parseThemeQuery(value: string | null | undefined): ThemeQueryMode | null {
+  if (!value) {
+    return null;
+  }
+
+  const normalized = value.toLowerCase();
+
+  if (normalized === "light" || normalized === "dark") {
+    return normalized;
+  }
+
+  if (normalized === "auto" || normalized === "system") {
+    return "system";
+  }
+
+  return null;
+}
 
 export function resolveStoredTheme(
   theme: Theme,