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/components/__tests__/Header.test.tsx

@@ -60,30 +60,8 @@ describe('Header', () => {
     expect(container.querySelector('.animate-spin')).toBeInTheDocument();
   });
 
-  it('shows active filter count badge', () => {
-    render(
-      <Header
-        {...defaultProps}
-        activeFilterCount={5}
-        showFilterToggle
-        onToggleFilters={vi.fn()}
-      />,
-    );
-    expect(screen.getByText('5')).toBeInTheDocument();
-  });
-
   it('renders brand name', () => {
     render(<Header {...defaultProps} />);
     expect(screen.getByText('Wrongmove')).toBeInTheDocument();
   });
-
-  it('shows mobile filter toggle when enabled', () => {
-    const onToggle = vi.fn();
-    const { container } = render(
-      <Header {...defaultProps} showFilterToggle onToggleFilters={onToggle} />,
-    );
-    // The filter toggle button has sm:hidden class
-    const filterButton = container.querySelector('.sm\\:hidden');
-    expect(filterButton).toBeInTheDocument();
-  });
 });