修复桌面凭据测试对构建环境的依赖

为桌面会话测试显式隔离默认服务器地址，避免持续集成环境影响未配置服务器场景的断言。
补充默认服务器地址下的凭据恢复验证，并确认注入默认地址时全部前端测试通过。

frontend/src/runtime/secureSessionStorage.test.ts

@@ -41,6 +41,7 @@ const createStorage = (): Storage => {
 
 describe("secure session storage", () => {
   beforeEach(() => {
+    vi.stubEnv("VITE_DESKTOP_SERVER_URL", "");
     vi.useFakeTimers();
     vi.setSystemTime(new Date("2026-07-02T00:00:00.000Z"));
     resetSecureSessionStorageForTests();
@@ -53,6 +54,7 @@ describe("secure session storage", () => {
   });
 
   afterEach(() => {
+    vi.unstubAllEnvs();
     vi.useRealTimers();
     resetSecureSessionStorageForTests();
     localStorage.clear();
@@ -158,6 +160,25 @@ describe("secure session storage", () => {
     expect(invokeMock).not.toHaveBeenCalled();
   });
 
+  it("restores credentials for the build default when no desktop server override is stored", async () => {
+    vi.stubEnv("VITE_DESKTOP_SERVER_URL", "https://default.ctms.example.com");
+    localStorage.removeItem(DESKTOP_SERVER_URL_KEY);
+    const token = createJwt(Date.now() + DESKTOP_SESSION_MAX_AGE_MS);
+    invokeMock.mockResolvedValue(JSON.stringify({
+      version: 1,
+      token,
+      storedAt: Date.now(),
+      expiresAt: Date.now() + DESKTOP_SESSION_MAX_AGE_MS,
+    }));
+
+    await initializeSecureSessionStorage();
+
+    expect(getSessionToken()).toBe(token);
+    expect(invokeMock).toHaveBeenCalledExactlyOnceWith("credential_get", {
+      serverOrigin: "https://default.ctms.example.com/",
+    });
+  });
+
   it("clears the previous server credential after a desktop server switch", async () => {
     const previousServerOrigin = "https://old.ctms.example.com/";
     const nextServerOrigin = "https://new.ctms.example.com/";