fix(frontend): preserve iframe theme across login

The iframe login used a fixed queryless redirect and called setTheme, which both dropped the iframe theme and persisted it into the normal site's shared theme storage.

frontend-web/src/core/embed/theme-mode.mjs

@@ -0,0 +1,19 @@
+/**
+ * @typedef {"light" | "dark"} EmbedThemeMode
+ */
+
+/**
+ * Map external theme names to the two themes supported by iframe surfaces.
+ *
+ * @param {string | null | undefined} rawTheme
+ * @returns {EmbedThemeMode}
+ */
+export function resolveEmbedThemeMode(rawTheme) {
+  const theme = (rawTheme ?? "").trim().toLowerCase();
+  return theme === "dark" ||
+    theme === "dark-blue" ||
+    theme === "深色" ||
+    theme === "深蓝"
+    ? "dark"
+    : "light";
+}