Fix window.matchMedia mock cleanup in header tests

[Copilot]

The window.matchMedia mock was overwritten in beforeEach but not restored in afterEach, which can leak state to other tests.

Changes

• Save original window.matchMedia before mocking (consistent with existing scrollIntoView pattern)
• Restore in afterEach hook

describe("Header", () => {
  let originalMatchMedia;
  
  beforeEach(() => {
    originalMatchMedia = window.matchMedia;
    window.matchMedia = matchMediaMock;
  });
  
  afterEach(() => {
    window.matchMedia = originalMatchMedia;
  });
});

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