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

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

tests/unit/utils/returnToEncoding.test.js

@@ -0,0 +1,21 @@
+// The Lucia project.
+// Copyright 2026-2026 DSP, inc.  All rights reserved.
+// Authors:
+//   imacat.yang@dsp.im (imacat), 2026/03/08
+
+import { describe, expect, it } from "vitest";
+import { encodeReturnTo, decodeReturnTo } from "@/utils/returnToEncoding";
+
+describe("returnToEncoding", () => {
+  it("round-trips ASCII paths", () => {
+    const original = "/discover/log/1/map?view=summary#node-2";
+    const encoded = encodeReturnTo(original);
+    expect(decodeReturnTo(encoded)).toBe(original);
+  });
+
+  it("round-trips Unicode paths without throwing", () => {
+    const original = "/files?keyword=流程圖#節點";
+    const encoded = encodeReturnTo(original);
+    expect(decodeReturnTo(encoded)).toBe(original);
+  });
+});