feat(briefwechsel): remove the standalone Briefwechsel view and its tests

Delete the /briefwechsel route in full (page, server load, eight
components and all co-located unit tests) and its end-to-end coverage
(briefwechsel-rows.visual, briefwechsel-a11y, the bilateral-correspondence
fixture, and the stale korrespondenz spec which targeted the route's
former /korrespondenz path). The card link now deep-links into document
search, so this view has no remaining inbound references.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/e2e/fixtures/bilateral-correspondence.ts

@@ -1,62 +0,0 @@
-import type { APIRequestContext } from '@playwright/test';
-
-/**
- * Test fixture for the briefwechsel row layout.
- *
- * Creates two persons and one document with sender/receiver between them so
- * that `/briefwechsel?senderId=X&receiverId=Y` navigates straight to the row
- * state (not the hero). Each seed uses a `Date.now()`-suffixed last name so
- * parallel runs and reruns never collide.
- *
- * The backend does not expose a person-delete endpoint, so only the document
- * is cleaned up in {@link cleanupBilateralPair}. The two timestamped persons
- * remain in the DB — acceptable for the test environment, and the unique
- * suffix means they cannot conflict with later runs.
- */
-
-export interface BilateralPair {
-	senderId: string;
-	receiverId: string;
-	documentId: string;
-}
-
-export async function seedBilateralPair(
-	request: APIRequestContext,
-	prefix: string
-): Promise<BilateralPair> {
-	const timestamp = Date.now();
-
-	const senderRes = await request.post('/api/persons', {
-		data: { firstName: prefix, lastName: `Sender-${timestamp}` }
-	});
-	if (!senderRes.ok()) throw new Error(`Create sender failed: ${senderRes.status()}`);
-	const senderId = (await senderRes.json()).id as string;
-
-	const receiverRes = await request.post('/api/persons', {
-		data: { firstName: prefix, lastName: `Receiver-${timestamp}` }
-	});
-	if (!receiverRes.ok()) throw new Error(`Create receiver failed: ${receiverRes.status()}`);
-	const receiverId = (await receiverRes.json()).id as string;
-
-	const docRes = await request.post('/api/documents', {
-		multipart: {
-			title: `${prefix} Brief`,
-			documentDate: '1950-06-15',
-			senderId,
-			receiverIds: receiverId
-		}
-	});
-	if (!docRes.ok()) throw new Error(`Create document failed: ${docRes.status()}`);
-	const documentId = (await docRes.json()).id as string;
-
-	return { senderId, receiverId, documentId };
-}
-
-export async function cleanupBilateralPair(
-	request: APIRequestContext,
-	pair: BilateralPair
-): Promise<void> {
-	// Only the document is purged — the backend has no person-delete endpoint
-	// and the timestamped last names make orphaned person rows safe to leave.
-	await request.delete(`/api/documents/${pair.documentId}`);
-}