test(briefwechsel): axe sweep at 3 viewports x 2 themes

Adds a dedicated axe-core sweep for /briefwechsel so contrast or
semantic regressions on the new row layout fail independently of
the catch-all accessibility suite. Scoped to the main landmark so
shared chrome violations (if any) aren't double-reported.

Refs #305

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/e2e/briefwechsel-a11y.spec.ts

@@ -0,0 +1,44 @@
+import AxeBuilder from '@axe-core/playwright';
+import { test, expect } from '@playwright/test';
+
+// Accessibility coverage specifically for the briefwechsel thumbnail-row
+// layout. Runs axe-core (wcag2a + wcag2aa) across three viewports and two
+// color schemes so color-contrast regressions on the meta text or the
+// distribution bar are caught independently of the site-wide sweep.
+
+const VIEWPORTS = [
+	{ name: 'mobile', width: 375, height: 812 },
+	{ name: 'tablet', width: 768, height: 1024 },
+	{ name: 'desktop', width: 1280, height: 800 }
+] as const;
+
+const THEMES = ['light', 'dark'] as const;
+
+test.describe('Accessibility — /briefwechsel row layout', () => {
+	for (const vp of VIEWPORTS) {
+		for (const theme of THEMES) {
+			test(`${vp.name} / ${theme} has no wcag2a/wcag2aa violations`, async ({ page }) => {
+				await page.setViewportSize({ width: vp.width, height: vp.height });
+				await page.emulateMedia({ colorScheme: theme });
+				await page.goto('/briefwechsel');
+				await page.waitForSelector('[data-hydrated]');
+
+				const results = await new AxeBuilder({ page })
+					.withTags(['wcag2a', 'wcag2aa'])
+					.include('main')
+					.analyze();
+
+				if (results.violations.length > 0) {
+					const summary = results.violations
+						.map((v) => `[${v.impact}] ${v.id}: ${v.description} (${v.nodes.length} node(s))`)
+						.join('\n');
+					console.log(
+						`\nAccessibility violations on briefwechsel (${vp.name}/${theme}):\n${summary}`
+					);
+				}
+
+				expect(results.violations).toEqual([]);
+			});
+		}
+	}
+});