Use UTF-8 safe return-to encoding and decoding across router and login

Co-Authored-By: Codex <codex@openai.com>

src/router/index.ts

@@ -12,6 +12,7 @@
 import { createRouter, createWebHistory } from "vue-router";
 import { useLoginStore } from "@/stores/login";
 import { getCookie, setCookie } from "@/utils/cookieUtil";
+import { encodeReturnTo } from "@/utils/returnToEncoding";
 import AuthContainer from "@/views/AuthContainer.vue";
 import MainContainer from "@/views/MainContainer.vue";
 import Login from "@/views/Login/LoginPage.vue";
@@ -182,7 +183,7 @@ function buildLoginRedirect(fullPath: string) {
   return {
     path: "/login",
     query: {
-      "return-to": btoa(fullPath),
+      "return-to": encodeReturnTo(fullPath),
     },
   };
 }