fix(e2e+i18n): add missing DE translation, fix E2E test selectors

- Add missing person_btn_conversations translation to de.json
- Fix birth/death year test: exclude /persons/new link + wait for hydration
- Fix lang test switching back to DE: wait for hydration + clear locale cookie
  (headless Chromium doesn't reliably delete cookies via document.cookie)

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

frontend/e2e/persons.spec.ts

@@ -55,8 +55,9 @@ test.describe('Person detail', () => {
 
 	test('birth and death year fields appear in edit mode and save correctly', async ({ page }) => {
 		await page.goto('/persons');
-		const firstPerson = page.locator('a[href^="/persons/"]').first();
+		const firstPerson = page.locator('a[href^="/persons/"]:not([href="/persons/new"])').first();
 		await firstPerson.click();
+		await page.waitForSelector('[data-hydrated]');
 
 		const editBtn = page.getByRole('button', { name: /Bearbeiten/i });
 		await editBtn.click();