refactor(frontend): delete dead conversations/ route (#193)

Remove the old conversations page that was superseded by briefwechsel/.
No navigation link pointed to /conversations; it was unreachable through
the UI. Deletes 5 files, removes 14 orphaned i18n keys from de/en/es
message bundles, and removes E2E tests that navigated to /conversations.

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

frontend/e2e/auth.spec.ts

@@ -24,7 +24,7 @@ test.describe('Authentication', () => {
 	});
 
 	test('protected routes redirect to /login without session', async ({ page }) => {
-		for (const url of ['/documents/new', '/persons', '/conversations']) {
+		for (const url of ['/documents/new', '/persons', '/briefwechsel']) {
 			await page.goto(url);
 			await expect(page).toHaveURL(/\/login/);
 		}