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/embed-session.ts

@@ -1,3 +1,7 @@
+import { resolveEmbedThemeMode } from "./theme-mode.mjs";
+
+export { resolveEmbedThemeMode } from "./theme-mode.mjs";
+
 /**
  * 「外部系统 iframe 套用深链」相关的**按标签页**会话开关。承载三个相互独立的能力，多由 URL
  * 路由参数开启（独立会话 `isolate` 另可由「外观」设置里的开关在运行时切换，见 `setIsolatedSession`），
@@ -85,16 +89,6 @@ export function readIsolateParam(search: string): FlagParam {
   return readFlagParam(search, "isolate");
 }
 
-/**
- * 把外部传入的主题色映射成离散明/暗。与 `resolveThemeFromQuery` 同源约定（dark-blue → 深色），
- * 另外宽松接受 `dark` / 中文「深色/深蓝」直传；其余（含空）一律浅色。
- */
-export function resolveEmbedThemeMode(raw: string | null | undefined): EmbedTheme {
-  const v = (raw ?? "").trim().toLowerCase();
-  if (v === "dark" || v === "dark-blue" || v === "深色" || v === "深蓝") return "dark";
-  return "light";
-}
-
 /** 取 URL query 里的 `theme` 值（embed 主题来源）。 */
 export function readEmbedThemeParam(search: string): string {
   return (toSearchParams(search).get("theme") ?? "").trim();