test: update AgentFooter tests to use document.querySelector for spinner checks

test: mock window.matchMedia in setupTests.js for consistent test environment

client/test/setupTests.js

@@ -20,6 +20,21 @@ import 'jest-canvas-mock';
 // Mock ResizeObserver
 import './resizeObserver.mock';
 
+// Mock window.matchMedia
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: jest.fn().mockImplementation((query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: jest.fn(), // deprecated
+    removeListener: jest.fn(), // deprecated
+    addEventListener: jest.fn(),
+    removeEventListener: jest.fn(),
+    dispatchEvent: jest.fn(),
+  })),
+});
+
 beforeEach(() => {
   jest.clearAllMocks();
 });