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

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

tests/router/routerGuard.test.js

@@ -4,6 +4,7 @@
 //   imacat.yang@dsp.im (imacat), 2026/03/06
 
 import { describe, it, expect, beforeEach } from "vitest";
+import { decodeReturnTo } from "@/utils/returnToEncoding";
 
 describe("router beforeEach guard logic", () => {
   beforeEach(() => {
@@ -70,7 +71,7 @@ describe("router beforeEach guard logic", () => {
       matched: [{ meta: { requiresAuth: true } }],
     });
     expect(result.path).toBe("/login");
-    expect(atob(result.query["return-to"])).toBe("/files");
+    expect(decodeReturnTo(result.query["return-to"])).toBe("/files");
   });
 
   it("allows requiresAuth route when refresh token can refresh session", () => {
@@ -100,7 +101,7 @@ describe("router beforeEach guard logic", () => {
       { refreshSucceeds: false },
     );
     expect(result.path).toBe("/login");
-    expect(atob(result.query["return-to"])).toBe(
+    expect(decodeReturnTo(result.query["return-to"])).toBe(
       "/discover/log/1/map?view=summary#node-2",
     );
   });