feat(i18n): add person_mention_search_prompt message key

For issue #380 — the new search input inside the @mention dropdown
needs an empty-state prompt distinct from "no results found".

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

frontend/messages/en.json

@@ -445,6 +445,7 @@
 	"person_mention_load_error": "Could not load person.",
 	"person_mention_loading": "Loading person…",
 	"person_mention_popup_empty": "No persons found",
+	"person_mention_search_prompt": "Enter a name…",
 	"person_mention_btn_label": "Link person",
 	"person_mention_create_new": "Create new person",
 	"transcription_editor_aria_label": "Transcription text",