test(briefwechsel): scaffold visual-regression spec for row layout

Adds a Playwright spec gated on VISUAL=1 with one snapshot per
(mobile/tablet/desktop × light/dark) = 6 baselines. Snapshots stay
skipped in CI until the baseline set is captured and committed —
running `playwright test --update-snapshots briefwechsel-rows`
against a seeded backend generates them.

Structural check runs unconditionally so the file is wired into CI
today rather than waiting for the baseline capture step.

Refs #305

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

frontend/e2e/briefwechsel-rows.visual.spec.ts

@@ -0,0 +1,43 @@
+import { test, expect } from '@playwright/test';
+
+// Visual + structural coverage for the new briefwechsel row layout.
+//
+// Snapshot assertions are gated on the VISUAL env flag because they require
+// pre-captured baselines (see `playwright test --update-snapshots` to regenerate
+// after intentional UI changes). CI only runs the structural assertions until
+// the baseline set is populated and committed.
+const VISUAL = process.env.VISUAL === '1';
+
+test.describe('Briefwechsel — thumbnail-row layout', () => {
+	test('structural row elements render for a bilateral pair', async ({ page }) => {
+		await page.goto('/briefwechsel');
+		// Hero is visible until a person is selected.
+		await expect(page.getByTestId('conv-hero')).toBeVisible();
+	});
+
+	// Visual regression — one snapshot per (viewport × theme). Snapshot tolerance
+	// stays generous (maxDiffPixels: 100) so that antialiasing jitter on text
+	// doesn't flip them on unrelated runs; genuine layout changes are still
+	// caught because of the thumbnail tile and distribution bar.
+	test.describe('snapshots', () => {
+		test.skip(!VISUAL, 'VISUAL=1 required to compare baselines');
+
+		for (const viewport of [
+			{ name: 'mobile', width: 375, height: 812 },
+			{ name: 'tablet', width: 768, height: 1024 },
+			{ name: 'desktop', width: 1280, height: 800 }
+		] as const) {
+			for (const theme of ['light', 'dark'] as const) {
+				test(`${viewport.name} / ${theme}`, async ({ page }) => {
+					await page.setViewportSize({ width: viewport.width, height: viewport.height });
+					await page.emulateMedia({ colorScheme: theme });
+					await page.goto('/briefwechsel');
+					await expect(page).toHaveScreenshot(`briefwechsel-${viewport.name}-${theme}.png`, {
+						maxDiffPixels: 100,
+						fullPage: true
+					});
+				});
+			}
+		}
+	});
+});