fix(dark-mode): use bg-header on layout header; set --c-header to brand-navy

- Change header --c-header dark value from #01335e to #012851 (brand navy):
  #01335e gave 4.3:1 with ink-3 (WCAG AA fail); #012851 gives 4.99:1 (pass)
- Switch header element from bg-surface to bg-header so dark mode uses the
  independent --c-header token instead of inheriting the surface background
- Fix both dark blocks (media query and manual override) to stay in sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/e2e/theme.spec.ts

@@ -70,8 +70,8 @@ test.describe('Theme toggle', () => {
 			const header = document.querySelector('header');
 			return header ? getComputedStyle(header).backgroundColor : null;
 		});
-		// --c-header in dark mode = #01335e → rgb(1, 51, 94)
-		expect(headerBg).toBe('rgb(1, 51, 94)');
+		// --c-header in dark mode = #012851 (brand navy) → rgb(1, 40, 81)
+		expect(headerBg).toBe('rgb(1, 40, 81)');
 	});
 
 	test('color-scheme is dark when data-theme=dark is set', async ({ page }) => {