chore: merge main into feat/issue-166 — resolve blue header conflicts

- +layout.svelte: adopt main's blue header structure (accent stripe, no
  border-b, bg-header instead of bg-brand-navy)
- layout.css light mode: drop --c-nav-active (removed by main); set
  --c-header: #012851 (confirmed correct now that header is brand-navy)
- layout.css dark mode: drop --c-nav-active; keep navy PDF tokens and
  --c-header: #012851 from our branch

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

frontend/e2e/header.spec.ts

@@ -0,0 +1,96 @@
+import { test, expect } from '@playwright/test';
+import AxeBuilder from '@axe-core/playwright';
+
+// #012851 — brand-navy, defined as --c-primary in layout.css
+const BRAND_NAVY = 'rgb(1, 40, 81)';
+
+test.describe('Header — brand-navy background', () => {
+	test('header background is brand-navy in light mode', async ({ page }) => {
+		await page.goto('/');
+		await expect(page.getByRole('navigation')).toBeVisible();
+
+		const bg = await page.locator('header').evaluate((el) => getComputedStyle(el).backgroundColor);
+		expect(bg).toBe(BRAND_NAVY);
+	});
+
+	test('header passes accessibility audit in light mode', async ({ page }) => {
+		await page.goto('/');
+		await expect(page.getByRole('navigation')).toBeVisible();
+
+		const results = await new AxeBuilder({ page }).include('header').analyze();
+		expect(results.violations).toEqual([]);
+	});
+
+	test('header background stays brand-navy after switching to dark mode', async ({ page }) => {
+		await page.goto('/');
+		await expect(page.getByRole('navigation')).toBeVisible();
+
+		await page
+			.getByRole('banner')
+			.getByRole('button', { name: /dark mode/i })
+			.click();
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+
+		const bg = await page.locator('header').evaluate((el) => getComputedStyle(el).backgroundColor);
+		expect(bg).toBe(BRAND_NAVY);
+	});
+
+	test('header passes accessibility audit in dark mode', async ({ page }) => {
+		await page.goto('/');
+		await expect(page.getByRole('navigation')).toBeVisible();
+
+		await page
+			.getByRole('banner')
+			.getByRole('button', { name: /dark mode/i })
+			.click();
+		await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
+
+		const results = await new AxeBuilder({ page }).include('header').analyze();
+		expect(results.violations).toEqual([]);
+	});
+
+	test('logo text is visible at 375px viewport', async ({ page }) => {
+		await page.setViewportSize({ width: 375, height: 812 });
+		await page.goto('/');
+
+		await expect(page.getByRole('banner').getByText('Familienarchiv')).toBeVisible();
+	});
+
+	test('hamburger menu opens on tablet viewport (768px)', async ({ page }) => {
+		await page.setViewportSize({ width: 768, height: 1024 });
+		await page.goto('/');
+		await expect(page.getByRole('navigation')).toBeVisible();
+
+		const hamburger = page.getByRole('button', { name: /menü öffnen/i });
+		await expect(hamburger).toBeVisible();
+		await hamburger.click();
+
+		await expect(
+			page.getByRole('navigation', { name: /mobile/i }).or(page.locator('#mobile-nav'))
+		).toBeVisible();
+	});
+});
+
+test.describe('Login page — AuthHeader', () => {
+	test.use({ storageState: { cookies: [], origins: [] } });
+
+	test('login page has brand-navy header with language switcher', async ({ page }) => {
+		await page.goto('/login');
+
+		const header = page.locator('header');
+		await expect(header).toBeVisible();
+
+		const bg = await header.evaluate((el) => getComputedStyle(el).backgroundColor);
+		expect(bg).toBe(BRAND_NAVY);
+
+		await expect(header.getByRole('button', { name: 'DE' })).toBeVisible();
+	});
+
+	test('login page header passes accessibility audit', async ({ page }) => {
+		await page.goto('/login');
+		await expect(page.locator('header')).toBeVisible();
+
+		const results = await new AxeBuilder({ page }).include('header').analyze();
+		expect(results.violations).toEqual([]);
+	});
+});