feat: support query theme bootstrap for embeds

lib/theme-sync.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, test } from "bun:test";
-import { resolveIncomingThemeMode, resolveStoredTheme } from "./theme-sync";
+import {
+  parseThemeQuery,
+  resolveIncomingThemeMode,
+  resolveStoredTheme,
+} from "./theme-sync";
 
 describe("theme sync helpers", () => {
   test("resolves stored analytics theme values", () => {
@@ -17,4 +21,14 @@ describe("theme sync helpers", () => {
     expect(resolveIncomingThemeMode("system", true)).toBe("dark");
     expect(resolveIncomingThemeMode("unknown", true)).toBeNull();
   });
+
+  test("parses theme override from query params", () => {
+    expect(parseThemeQuery("light")).toBe("light");
+    expect(parseThemeQuery("dark")).toBe("dark");
+    expect(parseThemeQuery("auto")).toBe("system");
+    expect(parseThemeQuery("system")).toBe("system");
+    expect(parseThemeQuery("LIGHT")).toBe("light");
+    expect(parseThemeQuery("unknown")).toBeNull();
+    expect(parseThemeQuery(null)).toBeNull();
+  });
 });