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/messages/de.json

@@ -182,5 +182,6 @@
 	"comp_taginput_placeholder_create": "Schlagworte hinzufügen...",
 	"comp_taginput_placeholder_filter": "Nach Schlagworten filtern...",
 	"comp_taginput_remove": "Schlagwort entfernen",
-	"comp_taginput_create_hint": "Enter drücken um Schlagwort zu erstellen."
+	"comp_taginput_create_hint": "Enter drücken um Schlagwort zu erstellen.",
+	"person_btn_conversations": "Konversationen anzeigen"
 }