fix: update tests for mobile responsive changes

- Add matchMedia polyfill to test setup (jsdom doesn't implement it)
- Remove Header tests for filter toggle (replaced by mobile FAB)

frontend/src/__tests__/setup.ts

@@ -1,5 +1,22 @@
 import '@testing-library/jest-dom/vitest';
 
+// Polyfill matchMedia for jsdom (used by useIsMobile hook)
+if (typeof window.matchMedia === 'undefined') {
+  Object.defineProperty(window, 'matchMedia', {
+    writable: true,
+    value: vi.fn().mockImplementation((query: string) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: vi.fn(),
+      removeListener: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+      dispatchEvent: vi.fn(),
+    })),
+  });
+}
+
 // Polyfill ResizeObserver for jsdom (used by Radix UI components)
 if (typeof globalThis.ResizeObserver === 'undefined') {
   globalThis.ResizeObserver = class ResizeObserver {