feat(person-mention): empty-state link to create the missing person

Leonie #5507 §5 + ReqEng #5510 §3: when the typeahead returned zero
results, the user was told their search failed and given no path to
recovery. Mirror PersonTypeahead's behaviour: offer a "Neue Person
anlegen →" link that opens /persons/new?name={query} in a new tab so
the transcriber doesn't lose their in-progress block.

Adds person_mention_create_new in de/en/es.

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

frontend/messages/de.json

@@ -425,6 +425,7 @@
 	"person_mention_load_error": "Person konnte nicht geladen werden.",
 	"person_mention_popup_empty": "Keine Personen gefunden",
 	"person_mention_btn_label": "Person verlinken",
+	"person_mention_create_new": "Neue Person anlegen",
 	"page_title_home": "Archiv",
 	"page_title_persons": "Personen",
 	"page_title_admin": "Administration",

frontend/messages/en.json

@@ -425,6 +425,7 @@
 	"person_mention_load_error": "Could not load person.",
 	"person_mention_popup_empty": "No persons found",
 	"person_mention_btn_label": "Link person",
+	"person_mention_create_new": "Create new person",
 	"page_title_home": "Archive",
 	"page_title_persons": "Persons",
 	"page_title_admin": "Administration",

frontend/messages/es.json

@@ -425,6 +425,7 @@
 	"person_mention_load_error": "No se pudo cargar la persona.",
 	"person_mention_popup_empty": "No se encontraron personas",
 	"person_mention_btn_label": "Vincular persona",
+	"person_mention_create_new": "Crear nueva persona",
 	"page_title_home": "Archivo",
 	"page_title_persons": "Personas",
 	"page_title_admin": "Administración",

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -217,7 +217,17 @@ const popupOpen = $derived(query !== null);
 			{#if loading}
 				<p class="px-3 py-2 font-sans text-sm text-ink-3">{m.comp_typeahead_loading()}</p>
 			{:else if results.length === 0}
-				<p class="px-3 py-2 font-sans text-sm text-ink-3">{m.person_mention_popup_empty()}</p>
+				<div class="flex flex-col gap-2 px-3 py-2.5">
+					<p class="font-sans text-sm text-ink-3">{m.person_mention_popup_empty()}</p>
+					<a
+						href="/persons/new?name={encodeURIComponent(query ?? '')}"
+						target="_blank"
+						rel="noopener"
+						class="font-sans text-sm font-medium text-brand-navy underline-offset-2 hover:underline"
+					>
+						{m.person_mention_create_new()} →
+					</a>
+				</div>
 			{:else}
 				{#each results as person, i (person.id)}
 					<div